© Reuters / Marko DjuricaA woman with a child casts her ballot inside a polling station in Kenya's town of Gatundu March 4, 2013.
Nairobi/Mombasa, Kenya - At least 15 people were killed in attacks by machete-wielding gangs on Monday as Kenyans queued to vote in a presidential election they hope will rebuild the country's image after a disputed 2007 poll unleashed weeks of tribal bloodshed.

A few hours before the 6 a.m. (0300 GMT) start of voting and with long queues across the nation, at least nine security officers in Kenya's restive coastal region were hacked to death, and six attackers were also killed, a regional police chief Aggrey Adoli said. The total toll had earlier been put at 17.

There were two separate attacks and senior police officers blamed one of them on a separatist movement - which, if confirmed, would suggest different motives to those that caused the post-2007 vote ethnic killings and could limit their impact.

Officials and candidates have made impassioned appeals to avoid a repeat of the tribal rampages that erupted five years ago when disputes over the poll result fuelled clashes between tribal loyalists of rival candidates.

More than 1,200 people were killed, shattering Kenya's reputation as one of Africa's most stable democracies and bringing its economy to a standstill.

As in 2007, the race has come down to a high-stakes head-to-head between two candidates, this time between Prime Minister Raila Odinga and Deputy Prime Minister Uhuru Kenyatta. Both will depend heavily on votes from tribal loyalists.

One of the machete attacks on Monday took place outside Mombasa and another in Kilifi about 50 km (80 miles) to the north. Senior police officers blamed the one near Mombasa on a separatist movement, the Mombasa Republican Council, which had sought and failed to have the national vote scrapped and a referendum on secession instead.

There was no immediate claim of responsibility and it was not possible to independently identify the attackers.

Even before the violence, many Kenyans were wary, particularly in places where it erupted last time. Shopkeepers have run down stocks and some people in mixed tribal areas have returned to their homelands elsewhere.

Bernard Otundo, 36, queuing quietly in Nairobi in the early morning darkness, said he expected a peaceful vote.

"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 have been a lot of awareness campaigns against violence and I don't think it will happen this time around, whatever the outcome."

Kenya's neighbors are watching nervously, after their economies felt the shockwaves when 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 veiled threats before the vote.

A grenade attack on a police post in Garissa, a city near Somalia, killed two civilians, inspector general of police, David Kimaiyo, told reporters. He also said a bomb exploded in the Mandera area, also near the border, causing no casualties.

He did not say who was behind the incidents and voters were undeterred, with long lines forming across the nation. In the early hours before voting, some Kenyans blew whistles and trumpet-like "vuvuzelas" to wake up voters. But others remain fearful that broader violence could flare.

"Our future is uncertain but we long for peace and victory is on our side this time round," said Odinga supporter 32-year-old Eunice Auma in Kisumu, a flashpoint after the 2007 vote.

"However, should our candidate (Odinga) fail to clinch victory. I'm afraid violence will erupt," she said.

Outgoing President Mwai Kibaki, barred from seeking a third five-year term, made what he described as a "passionate plea" for a peaceful vote. The candidates have pledged to accept the result. But the close race has raised the sense of uncertainty.

Though well ahead of six other contenders, polls suggest Odinga and Kenyatta will struggle to secure 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, while a narrow first-round win could raise prospects for challenges.

The West is fretting about the outcome because one leading candidate, Kenyatta, 51, has been indicted by the International Criminal Court with his running mate, William Ruto, for orchestrating the post-2007 vote violence. He denies the charges. But, if he wins, it would present a diplomatic dilemma for Western nations that are big aid donors to Kenya.

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.

Some voters still grumbled about the slow process as lines snaked hundreds of meters (yards) from the polling station. "People are beginning to fall and faint on the queue," said Peter Gichuchi, waiting for hours in the steamy heat of Mombasa.

To build confidence, 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.

Even so, Odinga, 68, has raised a warning flag, telling Reuters two days before the vote that the commission had by "design or omission" failed to register all voters in his strongholds, a charge the commission denies. On Monday, he condemned the violence.

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

Source: Reuters; Additional reporting by Hezron Ochiel in Kisumu, Drazen Jorgic, Beatrice Gachenge, Yara Bayoumy and George Obulutsa in Nairobi