Mikalay Statkevich and Dzmitry Vus challenged Belarus's authoritarian president in the December 19 election that international monitors called "flawed."
Both are charged with organizing mass disturbances and could face up to 15 years in prison if found guilty. They pleaded not guilty.
Prosecutors said they led an armed "mob" to smash various properties and "seize the government building" in Minsk, according to the French news agency AFP.
Several other opposition figures accused of organizing acts that disturbed public order also went on trial on May 11, and could face up to three years in prison.
Former Belarusian leader Stanislau Shushkevich described the hearings as little more than show trials.
"This is a political act," Shushkevich said. "This is a continuation of election fraud and injection of public fear, and the revenge on those who [actually] won the election."
Thousands of people took to the streets on December 19 to demonstrate against official results that handed Lukashenka a fourth five-year term in office and which the opposition said were rigged.
Statkevich and Vus join a total of 27 opposition figures -- including three other former presidential candidates -- currently being tried over those protests.
Another former presidential candidate, Ales Mikhalevich, fled the country and was granted asylum in Czech Republic earlier this year.
Fourteen opposition activists have so far been convicted and handed prison sentences of between two and four years.
Journalist Iryna Khalip and senior opposition figure Pavel Sevyarynets also pleaded not guilty today to a lesser charge of disrupting social order, and face a maximum of three years in prison. Several U.S. and European Union diplomats were present in the courtroom during Khalip's trial, according to AFP.
Khalip is the wife of Andrey Sannikau, another former presidential candidate whose trial is ongoing. Sannikau, who was badly beaten at the time of his arrest, placed second after Lukashenka according to the official results, taking home 2.5 percent of the vote to Lukashenka's 80 percent.
His trial, which began April 27, continued today. Sannikau faces up to 15 years in prison if convicted of organizing mass riots.
Belarusian authorities brutally dispersed the rally following the election and detained more than 600 protesters, prompting Western officials to impose sanctions and a travel ban on Lukashenka and more than 100 of his associates.
European Union foreign-policy chief Catherine Ashton referenced the trials today at the European Parliament in Strasbourg.
Lukashenka "left us with no other option than to adopt strong sanctions in response, targeting those in the regime responsible for the crackdown, including President Lukashenka himself, and also that we should step up our support for civil society and for people-to-people contacts," Ashton said.
More evidence of that crackdown emerged on May 10, when a court in Belarus ordered the opposition Belarusian National Front to leave its Minsk offices, according to AFP.
Senior party figure Ryhor Kastusyou -- who challenged Lukashenka in the election -- told the agency that the move could lead to the party's liquidation.
Meanwhile, three Russian human rights observers attending the trials were expelled from the country on May 6. The Committee on International Control over the Human Rights Situation in Belarus said its three representatives at the trials were arrested in Minsk and ordered to leave the country within 24 hours.
While the crackdown and the subsequent trials have received much attention in Western capitals, information in Belarus is harder to find.
"There is no information about it anywhere, even on the major websites," says a man in the western city of Hrodna. "I voted for Sannikau and I did not hide it. I'm sorry that everything ended this way. I think what's happening is wrong."
based on reports by RFE/RL's Belarus Service with additional local reports