Police say all the victims were suspected Sunni militants, including several foreigners.
The attack in the largely Shia city comes amid a wave of anger among Iraqi Shia over a bomb attack on one of their holiest shrines in Samarra.
A spokesman for Iraq's top Shia cleric Grand Ayatollah Ali al-Sistani said the anger may be hard to contain.
"You wouldn't expect an abrupt or sudden calm, because there are some people whose reaction you can't control," London-based spokesman Fadel Bahar al-Eloum told the BBC.
Gunmen
Basra police say the gunmen entered the Mina prison late on Wednesday, disarming the guards and seizing a group of suspected Sunni militants.
Their bodies were later found in the city, amid reports that they had been tortured before being shot.
Egyptian and Saudi nationals were among the victims, police said.
The gunmen were reportedly dressed as police officers and arrived at the prison in several vehicles.
The killings followed unrest in the city. There had been clashes between followers of Shia cleric Moqtada al-Sadr and Sunnis, and a Sunni shrine on the outskirts of Basra was said to have been burnt.
The offices of the Sunni Iraqi Islamic Party were also attacked.
Earlier on Wednesday, unidentified attackers blew up the dome of the Shia al-Askari shrine in Samarra.
"I hope that both sides acknowledge that whoever executed this act of religious terrorism is seeking only to fan the flames of hatred and spark civil war"
No group claimed responsibility for the bombing, although Iraqi television said several people had been arrested.
No-one appeared to have been injured in the attack.
The al-Askari shrine is one of two shrines in the city that attract Shia pilgrims from around the world.
Appeals
Following the attack, thousands of demonstrators gathered near the shrine, waving Iraqi flags and calling for justice.
Dozens of Sunni mosques in Iraq were later reported to have been targeted and at least eight Sunnis killed across Iraq in what appeared to be reprisal attacks.
President Jalal Talabani, a Sunni Kurd, has appealed for calm, and urged Iraqis to work together to avert "the danger of civil war".
British Prime Minister Tony Blair and US President George W Bush condemned the attack.
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