BAGHDAD, Iraq – A suicide bomber struck the Baghdad headquarters of Iraq’s biggest Shiite political party yesterday, killing three people, as the government announced plans to close borders and restrict movements to bolster security in the national election. Three candidates were slain as insurgents intensified their campaign to subvert the ballot.

The Cabinet member responsible for internal security urged fellow Sunni Arabs to disregard threats by Sunni extremists and vote in the January 30 election, in which Iraqis will choose a 275-member National Assembly and regional legislatures. Otherwise, the minister warned, the country will slide into civil war.

In a positive development, a Catholic archbishop kidnapped in northern Iraq was released yesterday without payment of ransom, the Vatican said. Archbishop Basile Georges Casmoussa, an Iraqi, said he believes he was kidnapped by mistake.

An American soldier was killed yesterday in a roadside bombing in Baghdad, and more foreigners were reported kidnapped, including Lebanese businessman Jebrail Adeeb Azar and eight Chinese construction workers. The Chinese were shown in a video being held hostage by gunmen claiming the captives worked for a company that deals with Americans. China’s official Xinhua News Agency said diplomats were “making all efforts to rescue” the hostages, who disappeared last week while traveling to Jordan.

In the attack on the Baghdad offices of the Supreme Council for the Islamic Republic in Iraq, one of the major groups contesting the election, a suicide driver detonated his vehicle after security guards stopped it at a checkpoint. The Shiite party, known as Sciri, has close ties to Iran and is strongly opposed by Sunni Muslim insurgents.

Iraqi police said the bomber and two others died, and nine people were wounded, including three police. The blast gouged a crater in the pavement, left several vehicles in flames and spread shredded debris on the street in the Jadriyah district.

“Sciri will not be frightened by such an act,” party spokesman Ridha Jawad said. “Sciri will continue the march toward building Iraq, establishing justice, and holding the elections.”

Sunni Muslim rebels, who make up the bulk of Iraq’s insurgents, have stepped up attacks on Shiites to frighten them into staying home on election day. Although many Sunni clerics and others oppose the election, Shiite leaders have told their followers that voting is their religious duty.

Shiites comprise about 60% of Iraq’s 26 million people and are expected to gain the political power long denied them by the Sunni Arab community, estimated at about 20%. Large turnouts are expected in the Shiite heartland south of Baghdad and in Kurdish-controlled regions of the north.

Insurgents have warned people to stay away from the polls and have threatened candidates. Gunmen shot and killed three candidates, officials said yesterday. Two of them belonged to Prime Minister Allawi’s political coalition, the Iraqi National Accord.

Alaa Hamid, who was running for the National Assembly, was killed Monday in Iraq’s second-largest city, Basra, an official said. Hamid was also the deputy chairman of the Iraqi Olympic Committee in Basra, which had been relatively quiet.

Riad Radi, who was contesting the local race for Basra’s provincial council, died Sunday when masked gunmen fired on his car as he was driving with his family, the official said.

The third candidate, Shaker Jabbar Sahla, was shot dead in Baghdad on Monday. He was a Shiite running for the National Assembly on the Constitutional Monarchy Movement ticket, headed by a cousin of Iraq’s last king.

American and Iraqi officials fear that a Sunni boycott could cast doubt on the legitimacy of a new government, heighten tensions between Shiites and Sunnis, and fuel the Sunniled insurgency.