Twenty-three immigration officers and 27 protesters were taken to hospital. One senior officer - Leung Kam-kwong, 43 - was among seven people in critical condition in Queen Mary Hospital early this morning.

Six mainland overstayers - Yeung Yee-ping, 29, Chow Ho-chun, 30, Chow Siu-ping, 28, Pang Hon-kwan, 41, and Wong Yan-chung, 36, and Lam Siu-sing, 26 - were in critical condition in various hospitals.

Twelve people were in serious condition, seven in stable condition and four in satisfactory condition. Twenty were treated and discharged.

The attack came after a four-hour stand-off following repeated attempts by the mainland abode-seekers to demand Hong Kong ID cards from immigration officials. It was the latest in a series of confrontations following a number of defeats in the courts for mainland migrants seeking the right to live in Hong Kong.

An immigration officer surnamed Choi said he was behind the group of migrants at the time of the explosion. He heard quarrelling then saw a sudden flash and fire spreading quickly through the floor.

'People started running in panic. I saw two immigration officers use a fire extinguisher to try to extinguish the blaze but it was too fierce.'

Government officials condemned the violence last night, warning those responsible would be prosecuted and promising security at the Immigration Department would be stepped up.

Visiting the scene about 30 minutes after the fire, Secretary for Security Regina Ip Lau Suk-yee and Director of Immigration Ambrose Lee Siu-kwong condemned the protesters.

Mrs Ip said she was 'shocked and angered by this irrational and violent action' and insisted the Government would not be pressured in its dealings with any residency claims.

Mr Lee said the same group of protesters had visited the offices at least 17 times over the past three months, demanding the department issue them identity cards. 'We have told them each time that we cannot help them and they must respect the rule of law,' he said. 'I feel very sorry and very sad for my staff.'

Chief Executive Tung Chee-hwa visited victims at Pamela Youde Nethersole Eastern Hospital last night. He said afterwards: 'What happened this afternoon was totally unacceptable and we are all very angered by what has happened. This sort of irrational behaviour is totally unacceptable to all of us.

'Those who do not have right of abode should not have any unrealistic expectations. We will execute the decisions of the courts. We want to uphold the rule of law in Hong Kong.'

Witnesses said the protesters arrived at the 13th-floor office of the Immigration Tower at about 2pm wanting ID cards but ignored officers who told them they should follow official procedures.

The group lingered in the lift lobby, making a disturbance. At 6pm when the office was about to close, officers again told them to leave. Protesters then held out a bottle of flammable liquid and threatened to light it if their demands were not met. The liquid was set alight when they were told one more time that ID cards would not be issued.

A huge fireball shot through the office of the investigation division. The fire was put out by the sprinkler system shortly afterwards. Eight fire engines and 22 ambulances arrived and ferried the injured to four hospitals on Hong Kong Island.

The sister of the man believed to have started the blaze said last night she regretted her brother had chosen 'a radical, senseless' act to push his demand.

'We have little idea what he did, but from time to time we heard on TV he would become violent. When we knew it, we reproached him. But when he's outside, he's on his own,' the woman said. 'He has been too stupid by far.'

A close friend of a critically injured migrant said: 'It is the Government who forced us to rebel against them.'

Condemnation poured in last night from political parties and human rights bodies over the incident. Liberal Party chairman James Tien Pei-chun said: 'The Government must take action to repatriate all the overstayers.'

The Democratic Party and the Democratic Alliance for the Betterment of Hong Kong both condemned the violence, with the DAB calling for the Government to set up a clear and fair application system for right of abode as soon as possible.

The Right of Abode Committee, a group representing the abode-seekers, last night issued a statement denying it was involved in the attack and calling on the Government to set up an independent taskforce to investigate.