Leaders of the station's serious crimes unit were suspected of involvement with local death squads, and seven were apprehended three days ago in British military raids, said Lt. Jenny Saleh of the British Royal Navy in Basra.
"We had intelligence to indicate that the serious crimes unit would execute its prisoners in the coming days, so we decided to intervene," Saleh said.
British officials said that after the first round of raids on Friday, the renegade police may have considered killing their prisoners. They did not elaborate.
British forces surrounded the station late Sunday night and launched the raid at 2 a.m. on Monday, an Iraqi police official said on condition of anonymity because of security concerns.
British troops were fired on as they approached the station, and killed seven gunmen, said Maj. Charlie Burbridge, a British military spokesman.
British and Iraqi forces transferred all 76 prisoners at the police station to another detention facility in downtown Basra, he said. Once the station was evacuated, British forces destroyed it with explosives.
In other developments:
One of the seven police leaders detained in Friday's raid by British forces was a leader with ties to militias, and without him present on Monday, British and Iraqi forces encountered "limited resistance" once they entered the station, Burbridge said.
Both raids were part of a larger British-Iraqi joint operation dubbed Operation Sinbad, aimed at "culling and rehabilitating" southern Iraqi police units of criminal elements, Burbridge said.
"We identified the serious crimes unit as, frankly, too far gone. We just had to get rid of it," he said.
An Iraqi security official said the government decided two days ago to eliminate the serious crimes unit and punish some officers.
"The interior minister decided to cancel the serious crimes unit in Basra city and replace it with a new one based inside the headquarters of Basra police," the official said on condition of anonymity because he was not authorized to speak to the media.
"The decision was made two days ago on the grounds of security violations by the serious crimes unit," the official said.
British troops have periodically conducted raids in an effort to root out corruption in the local police force, which has been infiltrated by Shiite militias.
Burbridge said the Iraqi police officials in this case were suspected of involvement in militias with criminal ties, rather than ones responsible for sectarian killings.
"It's very different here compared to Baghdad. They're criminal, local, political and they're tribal — but the one thing they are not is sectarian," he said.
Britain has 7,000 troops in Iraq, mostly based around the southern Shiite city of Basra. British officials have said they expect to withdraw several thousand troops from Iraq next year.
More than 120 British personnel have died in Iraq since the U.S.-led invasion in March 2003.