By Helen Fawkes
BBC News, Kiev
There was a sense of pride as Alla queued up to vote in Kiev.
Police water cannons are on standby in Kiev
Even before polling station number 62 opened, she joined the crowd gathered outside.
Inside the school gym which is acting as the voting hall, polling booths are surrounded by long curtains in blue and yellow, the colours of the national flag.
"I think the Ukrainian nation has risen from its knees and now we can freely elect our own president," she said.
The person who replaces the outgoing Leonid Kuchma will be only the third president of Ukraine since independence from the Soviet Union.
Thirty-seven million Ukrainians are eligible to vote and turnout has been high. There were queues at some of the polling stations in the capital.
One in five Ukrainians had cast their vote after just three hours of polling.
Fears of violence
The election campaign has been highly charged. There have been allegations of media bias, intimidation and even an alleged poisoning.
There have been concerns about possible attempts to rig the election, and some violations have already been reported. It has created an atmosphere of apprehension.
At polling station number 62, pensioner Anatoly cast his vote in a clear Perspex ballot box below a basketball hoop.
The huge ballot boxes were guarded by men in smart suits, watched over by observers.
Five police officers and other security men were also based at the building and tens of thousands of extra police officers were on duty nationwide.
"The campaign has been dirty, now there is all this nervousness, all this tension in the country it is bad for our democracy," Anatoly said.
The opposition and the authorities have accused each other of plotting violence.
Future at stake
There are fears of trouble outside the Central Election Commission in Kiev which is Ukraine's electoral authority.
Sitting in its car park, covered in camouflage netting, is an armoured personnel carrier.
There is more military hardware at the entrance to the Central Election Commission, and two police water cannons can be seen beyond the steel barricades that surround the Soviet looking building, also covered in green netting.
There have also been reports that tanks have been seen travelling between towns in other parts of Ukraine.
At stake in this election is the future direction of the country.
Voters can choose from more than 20 candidates on the long green ballot papers.
But only two men are seen as having a realistic chance of winning.
The key contenders are the opposition leader, Viktor Yushchenko, and the Prime Minister Viktor Yanukovych, who is backed by both the outgoing president and the Russian President Vladimir Putin.
Mr Yushchenko is seen as a reformer interested in pushing Ukraine closer to Western Europe, while Mr Yanukovych is considered to be more interested to cultivating closer ties with Russia.