Twin suicide bombers blew themselves up in a crowd of about 500 Iraqi police officers today, killing at least 20 people, as militants escalated their fight against government forces who have launched an operation to quash the insurgency in Iraq’s capital.

The attacks, in the largely Shia town of Hilla, south of Baghdad, came a day after Iraqi police fought pitched battles with insurgents as thousands of security forces backed by American troops began sweeping through Baghdad’s streets in Operation Lighting designed to catch militants responsible for killing more than 740 people since Iraq’s new government was announced in late April.

In a move that could further fuel a sectarian crisis, American troops detained the head of the Iraqi Islamic Party, Iraq’s largest Sunni Muslim political party, during a house raid early today in western Baghdad, a top party official and police said. Mohsen Abdul Hamid, the head of the Iraqi Islamic Party, was detained by American soldiers with his three sons and four guards, said the party's secretary-general Ayad al-Samarei. American military officials could not immediately confirm the detentions.

Mr al-Samarei accused American soldiers of raiding Mr Abdul-Hamid’s home and confiscating various items, including a computer. "This is a provocative and foolish act and this is part of the pressure exerted on the party," he said "At the time when the Americans say they are keen on real Sunni participation, they are now arresting the head of the only Sunni party that calls for a peaceful solution and have participated in the political process."

Mr Abdul-Hamid, aged in his late sixties, is regarded as a moderate Islamic leader. He was a member of the now dissolved American-appointed Iraqi Governing Council and has been involved with the party since the 1970s and headed it since 2003.

Mr Abdul-Hamid’s party had in recent weeks taken steps to become more involved in the political process following what essentially amounted to a boycott of political life by the group. Sunni Muslim Arabs are thought to make up the core of a virulent insurgency.

Iraq’s president, Jalal Talabani, condemned Mr Abdul-Hamid’s arrest and called for him to be set free, according to a statement released by his office. The statement did not identify who had detained Mr Abdul-Hamid.

"Iraqi President Jalal Talabani expressed his surprise and discontent about the arrest of the head of the Iraqi Islamic Party, Dr. Mohsen Abdul-Hamid, and called for his immediate release," the statement said. "The Iraqi president said that no one gave prior notice to the Presidential Council about the arrest of Dr. Mohsen Abdul-Hamid. This way of dealing with such a distinguished political figure is unacceptable."

The arrests came on the second day of Operation Lightning, the Iraqi-led anti-insurgent offensive in Baghdad that Mr Abdul-Hamid’s party opposes, believing security forces will trample over innocent people’s rights.

Operation Lightning, which is being backed by American forces, began on Sunday with the first of more than 40,000 soldiers and police searching hundreds of vehicles and raiding houses. It was launched following a wave of attacks, mostly carried out by suicide bombers, killed more than 740 people since the April 28 announcement of the new government, according to an Associated Press count.

Sunni Muslims were Iraq’s dominant community under Saddam Hussein, but they have lost influence since the dictator’s fall two years ago and the country’s predominant Shia community has gained political power.

A senior Sunni leader, whose charity this month joined forces with the Iraqi Islamic Party and the influential Association of Muslim Scholars to form a major Sunni political bloc, said Mr Abdul-Hamid’s arrest was part of a "huge campaign directed against Sunnis."

"Such acts will hamper our efforts to convince the Sunnis to participate in the political process," Adnan al-Duleimi, the head of the Sunni Endowment, said during a news conference.

The country’s raging insurgency is believed to be driven mainly from disaffected Iraqi Sunnis and extremist Islamists from neighbouring, predominantly Sunni Arab states. Tensions have been high in recent weeks during a spate of violence, some which has demonstrated Sunni-Shia tensions. Sunni and Shia religious leaders have been trading accusations against each other’s communities amid the killings of hundreds of people, including Shia and Sunni clerics.

The attacks in Hillah, took place at about 9:15 am (0615BST) when two suicide bombers wearing explosive belts blew themselves up in the middle of group of policemen demonstrating outside the governor’s office, staggering the blasts to maximise death.

Hillah was the site of the deadliest single attack since the fall of Saddam Hussein, a February 28 suicide car bombing against police recruits that killed 125 people.

In attacks that took place in Baghdad and other parts of Iraq, insurgents killed at least 30 people on Sunday, including a British soldier.

Abu Musab al-Zarqawi’s al-Qaeda in Iraq claimed responsibility for nearly all the attacks in internet statements that could not be independently verified.

Times Recommends

• Over the Wall
• Why Africa welcomes the ‘new colonialism’
• Which will come out on top: paper or gold?