The blast in the eastern Punjab province capital also left more than 70 people injured, two weeks after former Premier Benazir Bhutto was killed in a gun and suicide attack in Rawalpindi.

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

He said the attacker was on foot and detonated his explosives at 11.45 am (local time) on a busy thoroughfare near a police picket 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 last year'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. The condition of 15 of the injured was described as critical.

Interior ministry spokesman Javed Iqbal Cheema also 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.