The court is near to a police station and an office that issues driving licences. It was not clear who was behind the blast.
Quetta is the capital of Baluchistan province which has seen ongoing unrest. Residents of the province have been demanding a greater share of wealth from their natural resources.
There has been a spate of suicide attacks in Pakistan that intelligence officials have linked to groups operating from tribal areas, seen as supporting the Taliban and its battle against Nato forces in Afghanistan.
Owais Ghani, Baluchistan's governor, said that the root cause behind the increasing extremism in certain areas is fall out from the situation in Afghanistan.
Lawyers and officials surround wounded
colleagues at a hospital [Reuters]
Talking to a private TV channel, he said extremism has spread in Afghan society due to the three decades of anarchy and unrest in that country.
He added that Quetta suicide attack is the continuation of ongoing suicide attacks in the North-West Frontier Province and Islamabad.
Some extremist organisations willing to create unrest in the country are behind the blast, he said.
Security measures would be tightened in Baluchistan, Ghani said, adding that people's co-operation is a must for checking the extremists' activities.
Local authorities also suspect that pro-Taliban militants are targeting sensitive sites to undermine Pakistan's support of the US and its so-called "War on Terror".
The attacks followed a Pakistan army air strike on a base in Waziristan in mid-January.
Pakistan has been under mounting pressure from the US and Afghanistan to tackle Taliban sanctuaries on its territory.
Taliban leaders are widely believed to be operating from in and around Quetta, though Pakistan consistently denies their presence.