Nairobi - Kenyans were voting Tuesday in elections headlined by a knife-edge battle between incumbent Uhuru Kenyatta and his rival Raila Odinga that has sent tensions soaring in east Africa's richest economy.From first-time voters to those bent with age, from urbanites to ethnic Samburu warriors, thousands descended upon polling stations long before dawn to cast their ballots.Voting began relatively smoothly, with delays, technical hiccups and unexpected downpours slowing the process at some of the 41 000 polling stations, but the electoral commission (IEBC) said the situation was being dealt with."We shall extend the time of voting for the same amount of hours lost before opening," said IEBC chief Wafula Chebukati.At one polling station police fired tear gas to calm a crowd angry over delays, but voting quickly resumed."We would like to reassure you that our officers have been deployed to resolve technical issues... and we have been informed that voting is going smoothly," Chebukati said.The streets of normally traffic-choked Nairobi were deserted as voters flocked to polling booths. In the centre of the country Samburu draped in colourful beads were asked to leave their spears outside the polling station before entering to be identified by fingerprint.No stranger to violent polls All eyes are on the biometric voter identification and tallying system which failed massively in 2013 and is seen as crucial to a smooth election amid opposition accusations of a plot to rig the vote.Kenyans are no strangers to violent polls, and tensions have soared over fraud claims and the murder of an official in charge of the electronic voting system in the final days of campaigning.Tuesday's elections are taking place a decade after a shambolic vote - which foreign observers agreed was riddled with irregularities - sparked violence which left more than 1 100 people dead and 600 000 displaced.More than 150 000 members of the security forces - including wildlife, prison and forestry officers - have been deployed for polling day.The international community is also keeping a close eye on the election in a country considered a bastion of stability in east Africa and a key partner in the fight against the Al-Qaeda-linked Shabaab.Hundreds of foreign observers, including former US secretary of state John Kerry and former South African president Thabo Mbeki, as well as delegations from the European Union, are overseeing the election.'Democracy after all' "I voted Raila, because he will be so much better to us. But if he does not win, it's ok. It's a democracy after all. Really, there's no need for violence," said Tom Mboya, 43, who works in construction and voted in the capital's largest slum Kibera.Odinga, 72, the flagbearer for the National Super Alliance (NASA) coalition, is taking his fourth and likely final stab at the presidency. He claims elections in 2007 and 2013 were stolen from him."In the unlikely event that I lose I don't need a speech, I will just speak from my heart," Odinga said shortly before voting.Both he and Kenyatta cast their votes shortly before midday.The devolution of power to Kenya's 47 counties after a post-conflict constitutional reform means elections are now a complex affair, with citizens casting six different ballots.Several tight races for posts such as governor have seen tensions flaring at the local level.Dynastic rivalry The election is set to be the final showdown of a dynastic rivalry that has lasted more than half a century since the presidential candidates' fathers Jomo Kenyatta and Jaramogi Odinga went from allies in the struggle for independence to bitter rivals.The men belong to two of Kenya's main ethnic groups, Kenyatta from the Kikuyu, the largest, and Odinga from the Luo.Both have secured formidable alliances with other influential communities in a country where voting takes place largely along tribal lines.Kenyatta, 55, is seeking re-election after a first term in which he oversaw a massive infrastructure drive and steady economic growth of more than five percent."He has done a lot for the country and he must absolutely be re-elected. He has built a lot of infrastructure, like the SGR train (between Nairobi and Mombasa), he has created jobs," said Evelyn Sum, 32, dressed in an elegant brown coat.However Kenyatta is criticised for soaring food prices - with prices jumping 20 percent year-on-year in May - and massive corruption scandals on his watch."Life is more and more expensive, especially the flour and the sugar. That's not good for poor people like us, and we hope that Odinga will change this," said Rose Lida, 48, wrapped in a red Maasai blanket on the chilly morning.There are more than 19 million registered voters in the nation of 48 million. Half are aged under 35.Counting will begin immediately after voting ends at 17:00 and first results are expected around Wednesday. Officials have a week to release final results. A run-off is possible but seen as unlikely by pollsters, with six other, little-known presidential candidates garnering barely one percent of the vote between them.