There are long queues outside polling stations as Zambians vote in what is expected to be one of the country's most fiercely contested elections.There have been delays and clashes in the capital, Lusaka but observers say the vote has been relatively smooth.President Rupiah Banda is expected to face his strongest challenge from Michael Sata.Since the last vote in 2008, an extra one million people have registered to vote - many young and unemployed.High copper prices have boosted economic growth but many ordinary Zambians say they have not benefited.Thousands of policemen have been deployed to prevent violence and the sale of axes and other potential weapons has been banned during the election period.Mr Banda defeated Mr Sata by just 35,000 votes in the 2008 election, which sparked rioting by some opposition supporters in their urban strongholds.A police spokeswoman said order had been restored on Tuesday after trouble broke following reports that a man had been found with pre-marked ballot papers.Many people were also angry that polling stations opened late."The polling station should have opened at 6 o'clock, but by 8 o'clock we're still here and no-one is explaining what's happening," James Phiri, an unemployed 22 year old waiting to vote in Lusaka told the Associated Press news agency.A truck delivering ballot papers was blocked from entering the densely-populated township of Kanyama and some of its materials grabbed and strewn across the street.Election officials later denied the reports of pre-marked papers but admitted they were unsure how many ballots had been taken and whether they would be able to source more for that constituency.Some of Mr Banda's advertising hoardings were set on fire.Some 5.2 million people - the highest on record - have registered to vote in these presidential, parliamentary and local elections.Polls opened at 06:00 local time (04:00 GMT) and will close at 18:00, with the first results expected late on Wednesday.Chinese-funded growthChief European Union election observer in Zambia Maria Muniz De Urquiza said these problems had mostly been solved."So far, the reports we have is that everything is going in a peaceful manner, in a calm way," she told the BBC.The BBC's Mutuna Chanda in the capital, Lusaka, says the queues stretch for 2-3km in some of the polling stations he has visited.Crowds of youths chanting "We want change, we want change" greeted Mr Sata as he arrived to vote in Lusaka, Reuters news agency reports."If things are free and fair, we are very confident. What's most important is Zambians are very alert in this election," AFP quotes him as telling reporters after voting.Agnes Ngoma, 57, who has a farm nearby the capital, said Mr Banda's economic stewardship should be rewarded."I think the current government has performed very well, exceeded our expectations. There's a lot of infrastructure that's been built - roads, hospitals, schools. The economy is booming," she told AFP.Mr Banda has campaigned on this platform - the economic growth largely spurred by Chinese investments and the government's decision to scrap a windfall tax of 25% on mining companies.Mr Sata has promised to re-introduce the tax and to promote policies that will bring greater benefit to poor people.More than 60% of Zambians live on less than $2 a day.'Voters bussed in'On the eve of the vote police chief Francis Kabonde said his officers would arrest anyone carrying axes, machetes and other weapons.The election commission said it was confident the polls would be free and fair."What the commission has put in place is really a transparent system where there will not be any space or chance for anyone to manipulate," commission spokesman Chris Akufuna told reporters.Earlier, Mr Sata's the Patriotic Front (PF) accused Mr Banda's Movement for Multiparty Democracy (MMD) party of preparing fraud.The PF alleged that the MMD had tried to bus in people from neighbouring Malawi to vote.The PF said it had blocked the bus from entering Zambia but the MMD denied it had been involved in any attempt to rig the elections.Eight other candidates are contesting the presidency.Voters are also electing 150 members of parliament and more than 1,000 local councillors.