A MASSIVE suicide car bomb, apparently triggered in retaliation for the Government's campaign against Islamist insurgents in the north-west of the country, has rocked Pakistan's cultural capital, Lahore.

Pakistani policemen attempt to remove a trapped colleague from the rubble in Lahore. A suicide car bomb attack Wednesday flattened a police building in Pakistan's city of Lahore, killing 23 people in what the government branded revenge for an offensive against the Taliban. (AFP/Arif Ali) It is feared more than 30 people were killed and 200 injured by the blast at the Lahore headquarters of the Inter-Services Intelligence agency, Pakistan's premier spy agency.

The explosion near Mall Road, one of Lahore's busiest streets, destroyed a police building and left the area littered with debris and burnt-out cars.

It was the deadliest bomb attack in Pakistan this year, and there were fears last night the casualty toll would rise as emergency workers searched the rubble for victims.

No group had claimed responsibility last night.

Interior Minister Rehman Malik linked the attack to Taliban insurgents whom troops are battling in the Swat Valley. "Enemies of Pakistan who want to destabilise the country are coming here after their defeat in Swat," he said. "There is a war and this is a war for our survival."

Mr Malik said the attack would have no impact on the Government's operation to drive the Taliban out of Swat Valley.

"Let me assure you there will be no negotiations, no discussions, with the militants," he said "This operation will carry on until the last one is flushed out. There will be no compromise."

Pakistani President Asif Ali Zardari condemned the attack and ordered an investigation.

Officials said a car carrying several armed men pulled up in a street between offices of the emergency police and Inter-Services Intelligence. The attackers opened fire on guards apparently to clear the way for the suicide bomber, who drove the vehicle to the police buildings and detonated it.

Gun battles were reported in the vicinity for some time after the blast and police made several arrests. Two suspects were arrested by police near the blast site and paraded in front of TV cameras. Commandos were called in to help secure the area.

Police said grenades and a suicide jacket were recovered from the area after the blast, which left a large plume of white smoke over the city. Pakistan's other major cities - Karachi, Islamabad and Rawalpindi - were put on high alert.

Since the operation against Taliban insurgents in the Swat Valley and the adjacent districts of Buner and Dir in Pakistan's troubled North-West Frontier Province, Pakistanis have feared retaliatory terrorist attacks.

Yesterday's blast was the third major terrorist attack Lahore has suffered in the past three months. In March the Sri Lankan cricket team and match officials were ambushed by gunmen while travelling to Lahore's Gaddafi Stadium. This was followed by an eight-hour siege at a police academy on the outskirts of the city.

Before those attacks, Lahore was considered relatively safe. The latest attack, which took place a short distance from the provincial parliament building and near one of the city's most exclusive hotels, shows that despite increased security, terrorists are still able to strike almost anywhere in the country.

Meanwhile, the Pakistani military said security forces fighting Taliban guerillas in the north-western Swat Valley would clear the city of Mingora within two or three days.

Troops are making "considerable progress" in Mingora while "intense fighting" continues in the Kabal area, it said.

The conflict is forcing almost 130,000 people a day to flee, the UN said, while warning many others were unable to escape.