A suicide bomber in the eastern Pakistani city of Lahore has killed at least 21 people and wounded 60, most of them police officers.
The bomber targeted a group of police gathered outside the High Court building ahead of an anti-government protest, officials said.
He blew himself up after being approached by police.
The attack comes amid extreme political tension following the assassination of opposition leader Benazir Bhutto.
The blast happened just before noon local time (0700 GMT), when the courts are at their busiest.
Television footage showed a scene of devastation in the area in front of the High Court.
"The explosion was horrible and a number of people, including police officers, were lying dead on ground," a passer-by, Tariq Javed, told the BBC.
There was no immediate claim of responsibility.
Police constable Jameel Ahmed told the Reuters news agency that the attacker was a young man who had arrived outside the court on a motorbike.
27 Dec: Opposition leader Benazir Bhutto and 20 people killed in suicide attack in Rawalpindi
21 Dec: At least 50 dead in a suicide attack on a mosque in north-western Pakistan
24 Nov: Twin suicide car bombings kill at least 30 in Rawalpindi
19 July: More than 40 killed in three separate bomb attacks across the country
14 July: 24 soldiers killed in suicide attack on a military convoy in north-western Pakistan
28 April: Suicide bomber kills at least 22 people at a rally in North-West Frontier Province
"He parked his bike and walked to the police and blew himself up," Mr Ahmed said.
Police said the explosion was timed to cause maximum damage.
City police chief Malik Mohammad Iqbal said that all but one of the victims were policemen.
The officers were gathering outside the court building ahead of a protest by lawyers against the rule of President Pervez Musharraf.
Pakistan has seen a massive rise in suicide bombings since 2007.
According to the Interior Ministry, the country was hit by over 50 suicide attacks last year, which claimed at least 3,000 lives.
Government officials had warned of an increased risk of attacks in Lahore ahead of the Shia Muslim holy month of Moharram, which starts on Friday.