Kenyans lined up on Monday to vote in a presidential election that will test whether the east African nation can rebuild its image after tribal blood-letting followed a 2007 poll, but the killing of at least four policemen cast an early shadow.

A few hours before voting officially started at 6 a.m. (0300 GMT), four police officers deployed to keep the peace during the vote in the Mombasa port city area were hacked to death by a gang of machete-wielding youths, a senior officer said.

Officials and candidates have made impassioned appeals to avoid a repeat of the tribal bloodshed that erupted over the disputed result of the 2007 election, which killed more than 1,200 people and hurt Kenya's reputation as one of Africa's most stable democracies.

An official from the Independent Electoral and Boundaries Commission (IEBC) confirms an empty voting box before the start of presidential and parliamentary elections in Kisumu, 350 km (217 miles) west of Nairobi March 4, 2013. REUTERS

Ambrose Munyasia, chief of police intelligence in the Coast region, said he suspected the gang that attacked the police were linked to the separatist movement, the Mombasa Republican Council, which had sought and failed to have the national vote scrapped and to hold a referendum on independence.

Bernard Otundo, 36, who joined hundreds of others waiting in line before the sun rose over Nairobi, said he was confident the vote would be broadly peaceful.

"Some of us have been here as early as 2 a.m. this morning. I got here slightly after 3 a.m.," he said. "There has been a lot of awareness campaigns against violence and I don't think it will happen this time around, whatever the outcome."

Long lines also built up in other areas of the country.

People wait to cast their votes during the Kenyan general elections at the Ldergesi primary school in Archers Post, Isiolo County in northern Kenya March 4, 2013. REUTERS

Outgoing President Mwai Kibaki, barred from seeking a third five-year term, addressed Kenyans before the election saying: "I also make a passionate plea for all of us to vote peacefully.

Indeed, peace is a cornerstone of our development."

As in 2007, the race has come down to a high-stakes head-to-head between two candidates, Prime Minister Raila Odinga and Deputy Prime Minister Uhuru Kenyatta. Once again both will depend heavily on votes from loyalists from their tribes.
    
    WATCHING NERVOUSLY

Though well ahead of six other contenders, polls suggest neither Odinga nor Kenyatta may be able to command enough ballots for an outright victory in the first round. That could set the stage for a tense run-off tentatively set for April 11.

A narrow first-round victory for either candidate could raise prospects for legal challenges.

Kenya's neighbours are watching nervously, after their economies felt the shockwaves when the violence five years ago shut down trade routes running through east Africa's biggest economy. Some landlocked states have stockpiled fuel and other materials.

The United States and other Western countries are worried about the election in a country seen as a vital ally in the regional battle against militant Islam. Adding to election tensions, al Shabaab militants, battling Kenyan peacekeeping troops in Somalia, issued a veiled threat days before the vote.

People wait to cast their votes during the Kenyan general elections at the Ldergesi primary school in Archers Post, Isiolo County in northern Kenya March 4, 2013. REUTERS

The West also frets about the outcome of the presidential race.

Kenyatta, 51, has been indicted for crimes against humanity by the International Criminal Court after he was accused with his running mate, William Ruto, of instigating the post-2007 vote violence. He denies the charges. But, if he wins, it would present a diplomatic dilemma for Western nations that donate hundreds of millions dollars a year to Kenya.

"There are those who said that Uhuru and Ruto will not run because we are facing cases in Europe, but God has opened that road for us so that people can decide," Kenyatta told a rally in Nairobi's central park on Saturday, where thousands of supporters gathered, some chanting, "Uhuru, Uhuru".

Like other candidates, Kenyatta called on his supporters to vote peacefully and promised he would accept any result.

To try to prevent a repeat of the contested outcome that sparked the violence after the December 2007 vote, a new, broadly respected election commission is using more technology to prevent fraud, speed up counting and increase transparency.

This could lead to a swifter announcement of results, after delays in 2007 fuelled the crisis. Provisional figures may emerge within hours of polls closing, although the commission has seven days to declare the official outcome.

Kenya has passed a new constitution since 2007, police chiefs have deployed extra forces to maintain security and there is a more independent judiciary which commands greater respect.

Officials have appealed to candidates to raise any challenges in the courts and not on the streets.
    
URGING PEACE

Even so, Odinga has already raised a warning flag, telling Reuters that the commission had by "design or omission" failed to register all voters in his strongholds, putting him at a disadvantage, a charge the commission denies.

"We know and we hope there will be no rigging this time," Odinga, who narrowly lost the 2007 race, told a Nairobi stadium filled with supporters.

People wait to cast their votes during the Kenyan general elections at the Girgir primary school in Archers. REUTERS

"We are urging all our supporters to be peaceful because you are winning," said the 68-year-old, a veteran of Kenyan politics who may now have his last shot at the presidency.

Many Kenyans, hoping for a peaceful vote, say memories of the brutal killings by gangs armed with machetes, knives and bows and arrows are still fresh enough to deter a repeat. Many want to see a calm vote and a result to help Kenya's economy.

"By the end of the day, I want to see the youth with jobs," said Tonny Wamagata, 30 who works on a casual basis as a bus conductor. He lined up with many others at a polling station in a church on the outskirts of Nairobi.

Yet many are still wary, particularly in places where violence flared last time. Shopkeepers have run down stocks and some people in mixed tribal areas have returned to their homelands elsewhere, a few worried by threatening leaflets.

Michael Ochieng, in the tense city of Kisumu on the shores of Lake Victoria where businesses were looted five years ago, said reports of such leaflets in areas of the Rift Valley, another site of slaughter after 2007, did not bode well.

He spoke of growing animosity between Odinga's Luo tribe and Kenyatta's Kikuyu, in a contest where many voters will make their decision based on tribal loyalties not ideology.

Alongside the presidential race, there are hotly contested elections for senators, county governors, members of parliament, women representatives in county assemblies and civic leaders.

In the teeming slum of Kibera in central Nairobi, where Luos and Kikuyus live side by side, the sight on the election's eve of the orange T-shirts of Odinga supporters walking alongside the red ones of Kenyatta backers encouraged some.

But Samuel Kitai, 60, a grain miller who is neither Kikuyu nor Luo and whose store was looted in 2007, was taking no chances. Pointing inside his largely empty corrugated iron shack, he said: "I usually have maize here, rice and everything, but now you can see the store is empty."

REUTERS