Unprecedented protests broke out in the ex-Soviet country after Lukashenko, who has ruled the ex-Soviet state for 26 years, claimed re-election with 80 percent of the vote on August 9.
Opposition rival Svetlana Tikhanovskaya says she has won the vote but Lukashenko's security forces have detained thousands of protesters, many of whom accused police of beatings and torture.
Several people have died in the crackdown but Belarusians have been demonstrating across the country for nearly a month, with more than 100,000 people flooding the streets of the capital Minsk for three straight weekends.
Dozens of people including student protesters and journalists covering rallies were detained this week.
On Saturday, around 4,000 people took to the streets and more than 90 people were detained, the interior ministry said.
- 'Strong when united' -
Tikhanovskaya, a 37-year-old political novice, urged supporters to turn up for Sunday's "March of Unity" set to begin at 1100 GMT.
"Remember we are strong as long as we are united," she said in a short video address.
Tikhanovskaya contested the election after her blogger husband was jailed and barred from running along with several other prominent Lukashenko critics.
She left Belarus under pressure from authorities and took shelter in EU member Lithuania.
On Friday, Tikhanovskaya addressed a meeting of the UN Security Council by video link, calling for sanctions against those responsible for the alleged electoral fraud and rights violations.
The Baltic nations of Lithuania, Latvia and Estonia have blacklisted Lukashenko and 29 high-ranking officials in his administration but other members of the EU bloc appear reluctant to target the Belarus strongman personally.
Russia has said it will respond to any Western attempts to "sway the situation" and President Vladimir Putin has raised the possibility of sending military support.
Putin has been keen to unify Russia and Belarus, and Moscow has accompanied its recent offers of economic and military aid with calls for tighter integration.
Lukashenko has in the past ruled out outright unification and sought to play Moscow against the West but his options now are limited.
On Thursday, Lukashenko hosted Russian Prime Minister Mikhail Mishustin and replaced the chief of the KGB security service in what some analysts said might have been done under pressure from Moscow.
The moustachioed leader said Russia and Belarus had agreed on issues they "could not agree earlier" and he planned to "dot all the i's" with Putin in Moscow in the next few weeks.
Lukashenko made headlines when he claimed during a meeting with Mishustin that his security forces had intercepted German calls showing that Putin foe Alexei Navalny's poisoning with a Soviet-era Novichok nerve agent had been faked.
Belarusian state television broadcast the "intercept" in which a Mike in Warsaw and Nick in Berlin discuss Navalny's materials and call Lukashenko a "tough nut to crack."
Social media in Russia went berserk in mocking the Belarus leader and even some staunchly pro-Kremlin propagandists expressed embarassment.
Lukashenko also raised eyebrows last month when he brandished an assault rifle and had his 15-year-old son Nikolai appear next to him in a bulletproof vest while also weilding a weapon.
Some observers say Lukashenko wanted to curry favour with Moscow but was becoming a liability.
"No one knows what intercept Alexander Grigoryevich (Lukashenko) will record and publish tomorrow and where he will run with an assault rifle," wrote Kirill Martynov, politics editor at independent newspaper Novaya Gazeta.
bur-as/bmm