Four car bombs in and around the capital killed 16 people, most of them security personnel, in a swift response to Iraq's widest homegrown clampdown since the fall of Saddam Hussein more than two years ago.
Nine soldiers taking part in Operation Lightning died in a car bombing on Sunday at their roadblock just south of the capital, while two policemen were killed when a car bomber targeted their patrol in southwestern Baghdad.
In western Baghdad, a car bomb targeting police commandos killed three people and wounded 20, an Interior Ministry source said, adding that police then fought a firefight with men in the area.
An earlier bombing on Sunday near the Oil Ministry left two dead, while violence elsewhere claimed the lives of a British soldier and seven Iraqis.
Attacks nationwide have claimed the lives of about 700 people so far this month, after the swearing in of Iraq's first democratically elected post-Saddam government.
Meanwhile, an internet statement attributed to the group led by Abu Musab al-Zarqawi, has said: "Squadrons and brigades directed by the shaikh of the mujahadin Abu Musab al-Zarqawi on Sunday launched an operation ... planned and supervised by our shaikh."
The statement said the operation was a reply to the "aborted encirclement plan in Baghdad announced by the Iraqi ministers of defence and interior" - a reference to the security net expected to involve up to 40,000 Iraqi security forces that was launched the same day.
Sunni and Shia leaders are trying
to defuse sectarian tensions
It also claimed the deadly bombing at the roadblock outside Baghdad.
It was not possible to verify the authenticity of the statement, the latest in a series of sometimes conflicting messages about the health of the Jordanian-born fighter and his role in the anti-US violence.
Nevertheless, the government claimed it had already captured hundreds of fighters.
"Search operations and raids have allowed us to arrest 500 people and find arms caches in several houses," said spokesman Laith Kubba, adding that "we are expecting reactions but this will have no effect on the general course of the operation".
A Defence Ministry source said: "The army has set up fixed checkpoints around Baghdad as well as mobile controls, and raids have been launched in the city."
Defence Minister Saadun al-Dulaimi said the security cordon would soon be extended to the provinces as well.
"Search operations and raids have allowed us to arrest 500 people and find arms caches in several houses"
Iraqi Defence Ministry spokesman
"The objective is to pass from a defensive position to an offensive one and to put the capacities of defence and interior ministry forces to the best use."
However, AFP photographers reported on Sunday that many central Baghdad streets appeared much the same as in the past few days.
Earlier on Sunday, two of Iraq's most influential Shia and Sunni organisations agreed to try ease sectarian tensions in the country.
In an effort to ease the escalating sectarian tensions, officials from the Sunni Association of Muslim Scholars met representatives from the Badr Brigades, the military wing of Iraq's largest Shia party, the Supreme Council of the Islamic Revolution in Iraq.
Organised by the anti-US Shia cleric Muqtada al-Sadr, Sunday's gathering aimed to quell tensions that began earlier this month when the Sunni association's leader, Harith al-Dhari, accused the Badr Brigades of killing Sunnis and executing their clerics.
A number of Shia clerics were also killed.
The Brigades not only denied the charges, they accused the Sunni association of failing to condemn the violence, and of trying to "push Iraq into a sectarian conflict".
"We are all Muslims, and usually problems happen between one family. We want to solve them on the basis of Islamic brotherhood," a Sunni official, Isam al-Rawi, said.
Talabani (L) and Barzani are the
two main Iraqi Kurd leaders
"We overcame many obstacles. The two parties agreed to serve Iraq and to preserve its unity," al-Sadr official Abd al-Hadi al- Daraji said.
It was also announced on Sunday that Kurdish leader Massoud Barzani is to head an autonomous Kurdish region in northern Iraq after a deal with his longtime rival, Iraq's new President Jalal Talabani.
A Kurdish official said Talabani and Barzani had agreed that the latter should rule Iraq's three northern provinces for the next four years.
The regional parliament will also meet on 4 June for the first time since the January elections, the official added.