The United States was catapulted into a worldwide diplomatic crisis today, with the leaking to the Guardian and other international media of more than 250,000 classified cables from its embassies, many sent as recently as February this year.

At the start of a series of daily extracts from the US embassy cables - many of which are designated "secret" – the Guardian can disclose that Arab leaders are privately urging an air strike on Iran and that US officials have been instructed to spy on the UN's leadership.

These two revelations alone would be likely to reverberate around the world. But the secret dispatches which were obtained by WikiLeaks, the whistlebowers' website, also reveal Washington's evaluation of many other highly sensitive international issues.

These include a major shift in relations between China and North Korea, Pakistan's growing instability and details of clandestine US efforts to combat al-Qaida in Yemen.

Among scores of other disclosures that are likely to cause uproar, the cables detail:

• Grave fears in Washington and London over the security of Pakistan's nuclear weapons programme

• Alleged links between the Russian government and organised crime.

• Devastating criticism of the UK's military operations in Afghanistan.

• Claims of inappropriate behaviour by a member of the British royal family.

The US has particularly intimate dealings with Britain, and some of the dispatches from the London embassy in Grosvenor Square will make uncomfortable reading in Whitehall and Westminster. They range from serious political criticisms of David Cameron to requests for specific intelligence about individual MPs.

The cache of cables contains specific allegations of corruption and against foreign leaders, as well as harsh criticism by US embassy staff of their host governments, from tiny islands in the Caribbean to China and Russia.

The material includes a reference to Vladimir Putin as an "alpha-dog", Hamid Karzai as being "driven by paranoia" and Angela Merkel allegedly "avoids risk and is rarely creative". There is also a comparison between Mahmoud Ahmadinejad and Adolf Hitler.

The cables name countries involved in financing terror groups, and describe a near "environmental disaster" last year over a rogue shipment of enriched uranium. They disclose technical details of secret US-Russian nuclear missile negotiations in Geneva, and include a profile of Libyan leader Muammar Gaddafi, who they say is accompanied everywhere by a "voluptuous blonde" Ukrainian nurse.

The cables cover secretary of state Hillary Clinton's activities under the Obama administration, as well as thousands of files from the George Bush presidency. Clinton personally led frantic damage limitation this weekend as Washington prepared foreign governments for the revelations. She contacted leaders in Germany, Saudi Arabia, the Gulf, France and Afghanistan.

US ambassadors in other capitals were instructed to brief their hosts in advance of the release of unflattering pen-portraits or nakedly frank accounts of transactions with the US which they had thought would be kept quiet. Washington now faces a difficult task in convincing contacts around the world that any future conversations will remain confidential.

"We are all bracing for what may be coming and condemn WikiLeaks for the release of classified material," state department spokesman PJ Crowley said. "It will place lives and interests at risk. It is irresponsible."

The state department's legal adviser has written to Wikileaks founder Julian Assange and his London lawyer, warning that the cables were obtained illegally and that publication would place at risk "the lives of countless innocent individuals … ongoing military operations … and cooperation between countries".

The electronic archive of embassy dispatches from around the world was allegedly downloaded by a US soldier earlier this year and passed to WikiLeaks. Assange made them available to the Guardian and four other newspapers: the New York Times, Der Spiegel in Germany, Le Monde in France and El País in Spain. All five plan to publish extracts from the most significant cables, but have decided neither to "dump" the entire dataset into the public domain, nor to publish names that would endanger innocent individuals. WikiLeaks says that, contrary to the state department's fears, it also initially intends to post only limited cable extracts, and to redact identities.

The cables published today reveal how the US uses its embassies as part of a global espionage network, with diplomats tasked to obtain not just information from the people they meet, but personal details, such as frequent flyer numbers, credit card details and even DNA material.

Classified "human intelligence directives" issued in the name of Hillary Clinton or her predecessor, Condoleeza Rice, instruct officials to gather information on military installations, weapons markings, vehicle details of political leaders as well as iris scans, fingerprints and DNA.

The most controversial target was the leadership of the United Nations. That directive requested the specification of telecoms and IT systems used by top UN officials and their staff and details of "private VIP networks used for official communication, to include upgrades, security measures, passwords, personal encryption keys".

When the Guardian put this allegation to Crowley, the state department spokesman said: "Let me assure you: our diplomats are just that, diplomats. They do not engage in intelligence activities. They represent our country around the world, maintain open and transparent contact with other governments as well as public and private figures, and report home. That's what diplomats have done for hundreds of years."

The dispatches also shed light on older diplomatic issues. One cable, for example, reveals, that Nelson Mandela was "furious" when a top adviser stopped him meeting Margaret Thatcher shortly after his release from prison to explain why the ANC objected to her policy of "constructive engagement" with the apartheid regime. "We understand Mandela was keen for a Thatcher meeting but that [appointments secretary Zwelakhe] Sisulu argued successfully against it," according to the cable. It continues: "Mandela has on several occasions expressed his eagerness for an early meeting with Thatcher to express the ANC's objections to her policy. We were consequently surprised when the meeting didn't materialise on his mid-April visit to London and suspected that ANC hardliners had nixed Mandela's plans."

The US embassy cables are marked "Sipdis" – secret internet protocol distribution. They were compiled as part of a programme under which selected dispatches, considered moderately secret but suitable for sharing with other agencies, would be automatically loaded on to secure embassy websites, and linked with the military's Siprnet internet system.

They are classified at various levels up to "SECRET NOFORN" [no foreigners]. More than 11,000 are marked secret, while around 9,000 of the cables are marked noforn. The embassies which sent most cables were Ankara, Baghdad, Amman, Kuwait and Tokyo.

More than 3 million US government personnel and soldiers, many extremely junior, are cleared to have potential access to this material, even though the cables contain the identities of foreign informants, often sensitive contacts in dictatorial regimes. Some are marked "protect" or "strictly protect".

Last spring, 22-year-old intelligence analyst Bradley Manning was charged with leaking many of these cables, along with a gun-camera video of an Apache helicopter crew mistakenly killing two Reuters news agency employees in Baghdad in 2007, which was subsequently posted by WikiLeaks. Manning is facing a court martial.

In July and October WikiLeaks also published thousands of leaked military reports from Afghanistan and Iraq. These were made available for analysis beforehand to the Guardian, along with Der Spiegel and the New York Times.

A former hacker, Adrian Lamo, who reported Manning to the US authorities, said the soldier had told him in chat messages that the cables revealed "how the first world exploits the third, in detail".

He also said, according to Lamo, that Clinton "and several thousand diplomats around the world are going to have a heart attack when they wake up one morning and find an entire repository of classified foreign policy is available in searchable format to the public … everywhere there's a US post … there's a diplomatic scandal that will be revealed".

Asked why such sensitive material was posted on a network accessible to thousands of government employees, the state department spokesman told the Guardian: "The 9/11 attacks and their aftermath revealed gaps in intra-governmental information sharing. Since the attacks of 9/11, the US government has taken significant steps to facilitate information sharing. These efforts were focused on giving diplomatic, military, law enforcement and intelligence specialists quicker and easier access to more data to more effectively do their jobs."

He added: "We have been taking aggressive action in recent weeks and months to enhance the security of our systems and to prevent the leak of information."