Alexander Lukashenko, accused in the West of human rights violations in 12 years in power, is all but certain to easily defeat three challengers, two from the small liberal opposition.
Alexander Milinkevich, the main opposition hopeful, accuses Lukashenko of preparing to rig the vote. The European Union, which has sanctions in place against Belarussian officials, expressed concern over a "lack of freedom".
About 6000 people massed on the outskirts of the capital, Minsk, for a rock concert, waving the EU flag and the traditional red-and-white national colours banned by Lukashenko. Those in attendance chanted "Freedom!" and "Long Live Belarus!"
Milinkevich told supporters to take to the streets after the polls closed, as Ukrainians did in the 2004 Orange Revolution, to keep the count honest despite official warnings that public order violations would be treated as terrorism.
Fighting for freedom
Milinkevich said: "Today we should prove with our actions that we are fighting for freedom."
Under a banner with red letters reading "For Freedom", dozens of musicians backed the calls for peaceful protest.
Belarus youngsters wave
opposition flags in Minsk
Shopkeepers and waiters half-whispered the phrase "come to the square" to passersby on the streets of Minsk, which were brimming with police and soldiers.
Anonymous leaflets called for revolution, and mobile phones received competing text messages alternately encouraging protests or warning people against falling victim to bloodshed.
Milinkevich told a small morning rally: "We are going there with flowers, peacefully. We are against violence. We are going to the square to say no to falsification."
Lukashenko remains highly popular, particularly among the elderly and those outside the capital. He tells traditionally reserved Belarussians that his policies have ensured stability and a degree of prosperity not seen in nearby ex-Soviet states.
Accused by the West of rigging polls since the mid-90s, he has vowed to crush any attempt at mass protests, like those that unseated Moscow-backed establishments in Georgia and Ukraine.
Jose Manuel Barroso says
Belarus risks damaging EU ties
On Friday, he vowed to "wring the necks" of rivals who dared to violate public order.
Jose Manuel Barroso, the president of the European Commission, warned Lukashenko that more harassment of the opposition would further damage Belarus's ties with the European Union, which has already imposed a travel ban on Lukashenko and other officials.
"We are very concerned about some developments that we have seen recently, namely the lack of freedom, the harassment of the leaders of the opposition and other very negative developments," he said during a visit to Moscow.
Dozens of opposition activists have been fined and sentenced to short jail terms for public order offences. International observers and reporters have been denied entry or deported.
Lidyia Yermoshina, head of the Central Electoral Commission, said campaigning was fair and equal for all candidates and that attempts to pressure Lukashenko would backfire.
"I think our people will unite even more as a result and demonstrate their best quality - love of their motherland," the state news agency BelTA quoted her as telling international observers.
She said about 20% of voters had already cast ballots during advance voting - a practice denounced by the opposition as a vehicle for fraud.
Polls during the campaign say Lukashenko would probably win a vote without cheating and opposition chances remained slim.