Mr Lukashenko, 51, gained almost 83 percent of the vote from Sunday's presidential election, according to the central election commission. His nearest rival, Alexander Milinkevich, won six percent.
More than 10,000 opposition supporters gathered in the capital Minsk on Sunday, the city's largest rally in years, despite a blizzard, to protest Mr Lukashenko's imminent win.
Opposition groups have vowed to continue protesting on Monday and call for fresh elections.
"We will demand that this election be recognized as invalid, including by international organisations," Mr Milinkevich told his supporters from the steps of the trade unions' palace.
Western election monitors from the Organisation for Security and Cooperation in Europe (OSCE) will publish their findings later on Monday.
Earlier OSCE's assessments in the leadup to the vote indicate the findings will be highly critical.
Mr Lukashenko has ruled Belarus for 12 years, and before the ballot vowed to crush any demonstration, warning that his security forces would "break the neck immediately, like a duckling's".
Call for ballot annulment
Main opposition candidate Mr Milinkevich joined the impromptu rally holding a bouquet of pink carnations and telling supporters: "We have overcome fear."
The Interfax news agency reported that Mr Milinkevich demanded Sunday's election be annulled due to the alleged vote-rigging.
"We will demand that this election be recognised as invalid, including by international organisations," Mr Milinkevich told the crowd.
The rally directed its anger towards a giant television screen displaying official results, chanting "Lies! Lies!".
Traffic jammed main streets, as cars hooted horns in support of protesters.
Members of the crowd waved a red and white striped Belarus national flag that was abolished under Mr Lukashenko's rule.
The opposition would like to mirror people-power revolutions that forced out regimes in Georgia, Kyrgyzstan and Ukraine after rigged elections.
Security forces kept a low profile despite threats by the authorities to crush post-election protests.
President Lukashenko is popular with large segments of Belarussian society for maintaining economic stability and ensuring that pensions are paid on time.
Twelve-year rule
The 51-year-old president is headed for a third term in office of the former Soviet republic of 10 million people.
Mr Lukashenko was able to seek re-election thanks to a controversial 2004 referendum that abolished the constitution's two-term presidential limit.
Belarus, sandwiched between the European Union, Russia and Ukraine, is also part of a battle for influence between the West and Moscow in the former Soviet territories.
The opposition has received moral support from European governments as well as the US.
US President George W Bush has described Belarus as "the last dictatorship in Europe" and Western powers accuse Mr Lukashenko of making a fair election impossible.
But Mr Lukashenko lashed back at the US leader labeling President Bush as the "terrorist number one on our planet."
"If someone thinks I'm a dictator, good luck to him. But it's impossible to have a dictator in Europe. Only stupid people would think that," Mr Lukashenko said.
The country's KGB security service claims that would-be protestors are part of a Western-backed coup plot and could be executed or imprisoned for life as "terrorists."
Moscow has warned the West not to meddle. Russia supports Belarus' growing economy with cheap energy supplies and the two countries are also in continuing talks on forming a "union state."