BAGHDAD (AFP) - Iraq on Friday prepared for its first election since 2005 with security forces on high alert after gunmen killed candidates and campaign workers, raising fears of new violence ahead of polling day. The run-up to Saturday's poll had been relatively free of violence but the shooting of election contenders in Baghdad and in the cities of Baquba and Mosul, on Thursday night exposed the threat that such attacks could throw voting day into chaos. The elections " being held in 14 of the country's 18 provinces " are seen as a key test of Iraq's steadily improving stability, as US President Barack Obama looks to redeploy American troops to Afghanistan. Iraqi and US military commanders have in recent days warned that Al-Qaeda poses a threat to the elections. Campaigning for the vote officially ended at 0400 GMT on Friday and Iraq's borders will be closed at 10:00pm. Transport bans and night-time curfews will also be put in place as part of stepped up security measures. Saturday's vote is expected to see Sunni Arabs turn out in force in a reversal of the January 2005 parliamentary elections and is also being seen as a quasi referendum on the leadership of Prime Minister Nuri al-Maliki. The Shia premier has been striding out as an increasingly strong figure on the political stage as he has presented a secular national agenda in response to the sectarianism that has long gripped Iraq. Although Maliki is not standing, he has thrown his support behind a list of candidates that make up the State of Law Coalition. A cleric in the holy city of Najaf urged worshippers attending weekly Friday prayers to vote. "We appeal to the people; Sunnis, Shias, Arabs, Kurds and Turkmen to participate in the elections. Thursday's killings, however, highlighted the risk that sectarian hatred poses to voting day. The killing led local security officials to bring forward to Friday morning the planned 10.00 pm ban on cars, Major Shaban Dawad, of the Mosul police told AFP. The UN's special representative to Iraq Staffan de Mistura on Friday condemned the murders, describing them as "a terrible crime designed to attempt to disrupt the democratic process on the eve of the elections." Iraq's provincial councils are responsible for nominating the governors who lead the administration, and oversee finance and reconstruction projects. They control a combined budget of 2.4 billion dollars. Security forces remain under federal government control. Meanwhile, three bomb disposal police were killed and 20 colleagues wounded on Friday as a roadside device they were trying to disarm exploded in central Iraq, a police official said. The officers found the device in downtown Diwaniyah and took it to their station but it exploded as they tried to disarm the bomb, the official said. Diwaniyah has in the past seen clashes between Shia factions " notably pitting followers of radical anti-US cleric Moqtada al-Sadr against the rival Supreme Iraqi Islamic Council, member of the governing coalition in parliament.