* Attack on police building in mainly Shi'ite city
* Iraq on alert for bin Laden revenge attacks
By Habib al-Zubaidi
HILLA, Iraq, May 5 (Reuters) - A car bomber killed more than 20 people and wounded 80 on Thursday at a police building in the mainly Shi'ite city of Hilla, an official said, as Iraq braced for revenge attacks after U.S. commandos killed Osama bin Laden.
Iraq's army and police have been on high alert since American forces shot dead the al Qaeda leader and security officials said they had received intelligence that the Sunni Islamist group's Iraqi wing would carry out revenge attacks.
The suicide bomber rammed his car into the entrance of a police headquarters in the centre of Hilla, 100 km (60 miles) south of Baghdad, during a shift change at around 6:40 am (0340 GMT), when many police officers were outside the building.
"More than 20 were killed, 80 wounded. Four are missing. All of them are from the police because the explosion happened at one of the main police headquarters," Babil province Deputy Governor Sadeq al-Muhanna told Reuters.
Muhanna said a police expert had determined that the plastic explosive C-4 was used in the attack.
"Although it is too early to pin the responsibility on one party, suicide explosions are mostly done by al Qaeda and we expect (al Qaeda) is behind this explosion. We said before and we say it again, al Qaeda will not be finished by the killing of its leader," he said.
A hospital source in Hilla said the blast killed 25 and wounded 83. Iraqi officials often give conflicting tolls.
Iraq has been a major battlefield for al Qaeda since the 2003 invasion that toppled Saddam Hussein. Dozens of bombings and other attacks are still mounted each month, although U.S. and Iraqi officials say al Qaeda in Iraq has been severely degraded in recent years.
"These events happen on a daily basis in Iraq and nothing could prove that it has anything to do with the killing of bin Laden. These are routine events in Iraq. Security breaches, we are used to them," an Interior Ministry source said.
Attacks on the army and police are rising ahead of a full withdrawal of U.S. troops by Dec. 31, more than eight years after the U.S.-led invasion of Iraq.
Blast walls in front of the police headquarters in Hilla collapsed and the building was badly damaged, a Reuters reporter at the scene said.
Shops, houses and buildings nearby also were damaged.
"The negligence comes from Baghdad because we're always asking them to increase the number of our policemen, but there is no response," Kadhim Majeed Tuman, the head of the Babil provincial council, said.
Last month, gunmen laid siege to a provincial council headquarters in Saddam's hometown of Tikrit. Fifty-eight people were killed and at least 98 wounded.
On Monday four people were wounded when a bomb attached to a car exploded in Hilla. (Additional reporting by Waleed Ibrahim, Aseel Kami and Muhanad Mohammed in Baghdad; writing by Serena Chaudhry and Jim Loney; editing by David Stamp)