The dead included a police officer, while five police were among the 25 wounded, the Interior Ministry reported.
Police reinforcements had been called in to secure the theatre, which was being used to hold a public meeting discussing Iraq's draft constitution. Several legislators were at the event.
The attack happened just after 3pm (1110 GMT) on Saturday as the crowd was leaving the theatre, according to a police officer on the scene.
Eleven cars, including a police patrol cruiser, were destroyed by the blast, which shattered windows of nearby buildings.
In the town of Balad, 70km (40 miles) north of Baghdad, five family members - Hussein Mosleh, 55, his wife and their three children - were killed when a car bomb exploded as they drove by, police said.
Another eight civilians were killed when their car was hit by a roadside bomb in al-Dur, 155km north of Baghdad, police said. Three others were killed in a similar attack in Ishaki, 100km north of the capital.
A little-known Iraqi group says it was behind an attack on a convoy near Basra that killed two British security guards, according to a web posting.
"Thanks be to God - this morning ... an attack with an explosive device was carried out on a British convoy, killing two Britons," said a statement from the group calling itself the Imam Hussein Brigades. Its authenticity could not be verified.
British soldiers in Basra were not
affected by the attack
The group, whose name suggests it is Shia Muslim, has in the past claimed responsibility for the killing of three British soldiers and an Iraqi judge in southern Iraq.
The statement was posted on a site used by the main Iraqi insurgent groups, including the al-Qaida group led by Abu Musab al-Zarqawi.
The two killed were security contractors employed by Control Risks Group and both were British citizens, said Peter Stevenson, a spokesman for the London-based company.
Two Iraqi children were also wounded by the blast, which damaged two vehicles in the convoy, a British army officer in Basra said.
"No British forces were involved in the incident," another British military spokesman said.
Southern Baghdad attack
In southern Baghdad, several people were hurt when a bomb blew up near a US military patrol, the US military said.
"We did have some casualties," it said, but gave no details.
An Iraqi Interior Ministry official said at least one civilian was killed.
The official also reported that Imane Naji Abd al-Razzak, a top official at Iraq's Health Ministry, was abducted by armed men from her home in western Baghdad.
The bodies of a senior Baghdad airport official, his driver and an aide, were also found on Saturday with their throats slit. The trio had been kidnapped on Wednesday.
Maher Yassin Jassem, responsible for telecommunications at the international airport, and the two employees were found blindfolded with their hands bound in a western district of the capital. "They had also been tortured," an official said.
The new attacks came as police increased the death toll of a bomb attack on Friday at an army recruitment centre in the northern Iraqi town of Rabia to at least 48 killed and 58 wounded.
Most victims of the attack, claimed by the group of al-Qaida's Iraq frontman Abu Musab al-Zarqawi, were men waiting to join the new Iraqi army.
Meanwhile in Baghdad, the secretary-general of the National Dialogue Council in Iraq, Khalaf al-Olayan, has escaped an assassination attempt.
His car came under a hail of bullets on Saturday morning in central Baghdad, injuring one of his guards.
Al-Olayan said "governmental parties" were behind the attempt on his life.