Yemen's capital has been rocked by a night of deafening explosions and gunfire as troops loyal to the president, Ali Abdullah Saleh, battled with rebel tribesmen and defected soldiers for control of the city.
The violence was some of the fiercest in months and has prompted fears that the fate of the country's nine-month civilian uprising may be sliding into the hands of Yemen's fractious armed forces.
The fighting broke out around 11pm on Sunday with sporadic bursts of gunfire. By dawn a series of huge explosions had ripped through buildings in the north, echoing around the surrounding mountains.
Three teenage protesters were severely wounded when a hail of rockets thudded into Change Square – the tented shantytown in central Sana'a where thousands of demonstrators have been camped out since February calling for Saleh's resignation. It is the fourth time in the past month that shells have fallen on the camp.
"Their wounds are appalling," said Anas Noman, a third-year medical student volunteering in the camp's mosque, now a field hospital. "We've transferred one of them to a nearby hospital for an amputation of his leg."
It was unclear on Monday exactly how many had died as a result of the latest violence.
A brutal crackdown by security forces on a pair of mass rallies over the weekend has turned into a full-blown military confrontation between the Republican Guard – an elite force headed by Saleh's son Ahmed – and the 1st armoured division, headed by General Ali Mohsin al-Ahmar who defected to the opposition in March. On Sunday night the two sides started firing mortars and anti-aircraft missiles at each other's military bases, sending plumes of black smoke rising above the city.
"This is far more complex than just unarmed protesters versus heavy handed government troops, it's now a power struggle between elite military factions," said Abdullah al-Faqih, a professor of politics at Sana'a University. "Unfortunately it is the unarmed protesters who are caught in the middle and are paying the price for the violence."
The most intense fighting took place in the northern neighbourhood of Hasaba, where a group of heavily armed rebel tribesmen are battling to defend the official residence of their influential leader, Sadeq al-Ahmar, who is also backing the opposition.
Ali Hussein, the owner of a small grocery shop on the southern outskirts of Hasaba, said that shells and bullets were "falling like rain" on people's houses. "Even the lampposts have bullet holes in them," he said.
Sana'a is nestled between four mountains all of which are controlled by government troops. Locals say Saleh has been using the vantage point to launch rocket attacks on his opponents.
The surprise return of Saleh in late September, who had been recovering in Saudi Arabia from an assassination attempt three months earlier, has plunged the country into a deeper state of uncertainty and sharpened the differences between pro and anti-government camps. His supporters feel more emboldened with him here, his opponents all the more determined to topple him.
Protesters in Change Square have started scribbling their names and dates of birth across their chests before they march so that their bodies can be identified if they are killed.
Hours after the explosions died down, thousands of women took to the streets and marched on the foreign ministry demanding UN intervention in the ongoing unrest. The march was prompted by the death of a 20-year-old woman, who was shot dead in a sniper attack in the southern city of Taiz on Sunday. She is the first female protester to die in the uprising.
The UN security council is expected to vote on Tuesday on a British-drafted resolution that would call for Saleh's immediate resignation in return for immunity from prosecution.
In a televised speech on Sunday, Saleh described the opposition as "insane people, who can't sleep and only want to take power", saying that will only happen through elections. He also claimed that Yemen was witnessing a "military-Islamic coup". "We have strong proofs of the co-operation between the Muslim Brotherhood and al-Qaida," Saleh said.