Baghdad - Insurgents in Iraq intent on derailing elections due in less than two weeks stepped up a campaign of violence across the country yesterday, claiming dozens more lives in shootings and car bombings.

A campaign of assassinations has claimed victims from north to south Iraq. Gunmen are now setting up their own checkpoints on most roads leading out of Baghdad.

Yesterday a suicide car bomber drove into the police headquarters in the oil refining town of Baiji, 100 miles north of Baghdad, and killed at least 10 people in the blast. About 30 more people were injured, and witnesses described seeing several burned corpses lying on the ground in the police compound.

Another eight Iraqis, all national guardsmen, were shot dead in an attack on their checkpoint outside a provincial broadcasting centre in Buhriz, near Baquba, also north of Baghdad. Later a militant group apparently led by the Jordanian militant Abu Musab al-Zarqawi claimed responsibility.

Six bodies were found in the western city of Ramad with notes attached to them describing them as collaborators. "The fate of every agent will be slaughter," one of the notes said.

The Catholic archbishop of Mosul was kidnapped at gunpoint yesterday. Archbishop Basile Georges Casmoussa, 66, of the Syrian Catholic church was seized by gunmen outside his church.

The number of attacks across the country now averages 80 a day, the same level as last spring when the US occupation was facing its greatest challenge, trying to head off armed uprisings in Sunni and Shia areas.

There is also a growing number of incidents south of Baghdad, even in previously quiet areas. Gunmen opened fire on a polling station in Musayib, 50 miles south of the capital. At least one guard and one insurgent were killed.

In the southern port city of Basra, mortars were fired at three schools that have been designated as voting centres. No one was injured but the schools were badly damaged. An additional 650 British troops from the Royal Highland Fusiliers arrived in the city on Sunday to boost security.

In the southern town of Numaniya, near Kut, gunmen shot dead the son of Habib Salman al-Katib, a representative of Grand Ayatollah Ali al-Sistani, the leading Shia clerical authority in Iraq. Several of his aides have been assassinated in recent days.

There have been other recent killings near Kut, once a peaceful Shia town, with several accounts of gunmen shooting drivers dead at checkpoints. At least 17 people died around the town in attacks on Sunday, including Iraqi policemen, national guardsmen, local government officials and Iraqis working for foreign companies involved in reconstruction projects.

Salama al-Khafaji, a high-profile moderate Shia politician, survived an ambush in Baghdad on Sunday - the second attempt on her life in the past year. Yesterday she cancelled plans for an election campaign tour through the south.

Iraqi officials have tried not to publicise the location of polling stations for fear of attacks. But the secrecy surrounding the election often goes much further. Although there are more than 100 political parties and coalitions taking part, few have given the names of candidates.

A security clampdown will be enforced in the days leading up to the election.

Yesterday the most senior American commander in Iraq, General George Casey, accepted it was likely to be a violent polling day. "The enemy we're fighting is not 10ft tall, but he's resourceful and persistent. Is there going to be violence on election day? There is."

Iraqi exiles in 14 countries began registering yesterday to take part in the election. Estimates of the number of expatriates entitled to vote range from one million to four million. Britain, with an estimated 150,000 eligible voters, has three centres, in London, Manchester and Glasgow.