A pre-dawn attack on police in Kenya early Monday killed several officers hours before Kenyans began casting votes in a nationwide election being held five years after more than 1,000 people died in election-related violence.

Police in the coastal city of Mombasa reported a 2 a.m. attack by a gang of dozens; early reports indicated several officers _ perhaps four or five _ and several attackers were killed. Police didn't immediately confirm a death toll.

The country's leaders have been working for months to reduce election-related tensions, but multiple factors make more vote violence likely. The police said late Sunday that criminals were planning to dress in police uniforms and disrupt voting in some locations.

In addition, intelligence on the Somali-Kenya border indicated Somali militants planned to launch attacks; a secessionist group on the coast threatened _ and perhaps already carried out _ attacks; the tribes of the top two presidential candidates have a long history of tense relations; and 47 new governor races are being held, increasing the chances of electoral problems at the local level.

Perhaps most importantly, Uhuru Kenyatta, one of two top candidates for president, faces charges at the International Criminal Court for orchestrating the 2007-08 postelection violence. If he wins, the U.S. and Europe could scale back relations with Kenya, and Kenyatta may have to spend a significant portion of his presidency at The Hague. Kenyatta's running mate, William Ruto, also faces charges at the ICC.

Long lines began forming early across the nation. In Kibera, Nairobi's largest slum, some 1,000 people stood in several lines at one polling station before daybreak. Voter Arthur Shakwira said he began standing in line at 4 a.m. but left the queue over confusion about which line to stand in. Nearby, Amos Achola said he arrived at the polling station at 2 a.m. and was one of the first to vote.

Kenyatta, a Kikuyu who is the son of Kenya's founding president, faces Raila Odinga, a Luo whose father was the country's first vice president. Polls show the two in a close race, with support for each in the mid-40-percent range. Eight candidates are running for president, making it likely Odinga and Kenyatta will be matched up in an April run-off, when tensions could be even higher.

Most voters in Kibera, like Achola, support Odinga.

"I think he wins but if he doesn't win I'll abide by the outcome," Achola said. "The other guy is also a Kenyan. If Kenyatta wins I'll accept it but I won't like. But I don't want violence."

New technology _ in part to prevent the allegations of rigging that haunted the 2007 vote _ appeared to slow down early voting. At the Mutomo Primary School in Gatundu, where Kenyatta is expected to cast his ballot, voting officials seemed overwhelmed by the finger-print technology. The election worker behind the computer looked nervous and sometimes scratched his head.

The first person to vote, an eldery woman, cast her ballot at 6:25 a.m., 25 minutes after the polls opened.

In Mombasa, police boss Aggrey Adoli said that police were attacked at 2 a.m. by a marauding gang while on patrol. He didn't immediately confirm a death toll but reporters at the scene said police indicated that up to five officers and several attackers were killed in a fight that involved guns and machetes.

A late Sunday attack in the city of Garissa, near the Somali border, killed two people, including a Red Cross paramedic and a driver. Officials said a candidate for parliament had been the target but was not hit.

Garissa County Commissioner Mohamed Ahmed Maalim said Sunday that officials intercepted communications that indicated terror attacks were planned. Maalim said soldiers are patrolling the region to prevent attacks from al-Shabab, the al-Qaida-linked Somali militant group. He said 300 specialized troops known as GSU are patrolling the Dadaab refugee camp, where more than 400,000 Somalis live.

In the weeks leading up to Monday's vote, described by Odinga as the most consequential since independence from the British in 1963, peace activists and clerics have been praying that this time the election is peaceful despite lingering tensions.

Odinga's acrimonious loss to President Mwai Kibaki in 2007 triggered violence that ended only after the international community stepped in. Odinga was named prime minister in a coalition government led by Kibaki, with Kenyatta named deputy prime minister.

Some 99,000 police officers will be on duty during an election in which some 14 million people are expected to vote.

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Associated Press reporter Daud Yussuf in Garissa contributed to this report. Rodney Muhumuza reported from Gatunda.