Opposition leader Alexander Milinkevich led about 6,000 chanting, flag-waving protesters in the Belarussian capital Minsk in deriding the country’s autocratic rule.
He said: “We'll destroy him in accordance with the constitution. We know how to do that. The law is on our side."
Milinkevich, who officially garnered just a tiny percentage of votes at a presidential election last month, urged the crowd to maintain the momentum achieved during an unprecedented week of protest that followed the election.
He also thanked neighbouring European countries for their support. "The election was a farce. We've come out to rise up off our knees and escape our fear... We will go out with leaflets, we will use the internet. We will go again to the countryside, where there are good people," Mr Milinkevich told the crowd, to chants of "Freedom!" and "Long live Belarus!"
Highly charged event
While the focus of commemorations was at the site of the power station in neighbouring Ukraine that exploded on April 26, 1986, the anniversary is a highly charged event in Belarus because its territory lay in the immediate path of the radioactive cloud that spewed from the site.
The nation's leadership has sought to repopulate areas abandoned in the wake of the accident and critics say that the authorities are ignoring serious health risks in doing so.
Nearly a quarter of Belarus' territory was contaminated, according to official Belarussian statistics, leading to high rates of thyroid cancer, particularly among children, and the resettlement of tens of thousands of residents.
President Lukashenko has made rehabilitating affected areas a priority, a cause that is broadly in line with recent controversial assertions by the World Health Organisation (WHO) that the accident's direct health consequences have been exaggerated.
The state's goal is "sustainable development of afflicted regions" and "multi-faceted rehabilitation of living conditions for people in the contaminated areas," Mr Lukashenko said in an April 19 statement.
“The president said yesterday that everything's clean now, that you can drink the milk... I'm shocked by the behaviour of our government," one scientist who was involved in the clean-up at Chernobyl, Ivan Nikitchenko, told the gathered crowd.
Mr Lukashenko was due to tour areas near the Ukrainian border, with his main focus on "developing clean agricultural production and provision of services to the population, the provision of specialised medical help and skills for medical institutions, as well as strengthening rural schools," the official Sovetskaya Belarussia newspaper said.
Protesters’ fears
The anniversary has long been an occasion for sizeable demonstrations in the capital against such policies, protests that the authorities have often met with force.
A number of students present said they feared they could be expelled from their places of education for participating in the demonstration, as happened to several who attended demonstrations last month.
"We want the authorities to pay attention to the problems of Chernobyl," said Vitaly, a student who was wrapped in a traditional red and white Belarussian flag that Lukashenko has abolished and replaced with a red and green one.
A sizeable security operation accompanied the protest, which took place at the Academy of Science because authorities barricaded off the central October Square - the focus of last month's protests.
Uniformed police filmed activists at close quarters with hand-held cameras and two large green prisoner trucks drove alongside protestors who had tried to gather on October Square, as they marched down the main street to the Science Academy.
This year's demonstration is seen as a test of the opposition's strength after it mounted an unprecedented week-long protest against Mr Lukashenko's re-election last month for a third term in office.