At least 22 Pakistani riot policemen and four civilians were killed and over 65 injured when a suicide bomber blew himself up near the Lahore [Images] High Court ahead of a planned protest by lawyers on Thursday.

"It was a suicide attack," said Malik Iqbal, police chief of Lahore, the capital of eastern Punjab province of Pakistan.

He said the attacker was on foot and detonated his explosives at 11.45 am (local time) near a picket set up by policemen at the General Post Office Chowk close to the Lahore High Court ahead of the lawyers' protest march to press for reinstatement of judges sacked during 2007's emergency.

A red alert was sounded in Pakistan's four provinces in the wake of the blast and security was stepped up around key government buildings and vital installations in the federal capital of Islamabad and the nearby garrison city of Rawalpindi.

Four passers-by and 22 policemen died in the blast, which occurred two weeks after former premier Benazir Bhutto [Images] was killed in a gun and suicide attack in Rawalpindi.

The condition of 15 of over 65 injured in the blast was described as critical. Interior ministry spokesman Javed Iqbal Cheema said the policemen were the target of the suicide bomber.

Around 100 riot policemen had been deployed at the square as lawyers were planning to organise a protest march seeking reinstatement of deposed judges.

The lawyers were holding a meeting inside the Lahore High Court building at the time of the blast and were safe, witnesses said. The protest was put off following the blast.

No group claimed responsibility for the attack, but the Pakistan government has blamed a string of suicide attacks against the security forces on pro-Taliban militants from the country's restive northwestern tribal areas.

A Geo News correspondent, who was at the site to cover the protest by the lawyers, reported witnessing a massive blast and huge ball of flame.

He said he had seen the severed head and legs of the suspected suicide bomber. The head was found almost 200 metres from the site of the blast.

Police boots and riot gear lay amidst pool of blood at the spot, and a policeman broke down and wept bitterly as his injured and dead colleagues were taken away by ambulances.