But their numbers were smaller than on election night and prospects for a Ukraine-style "Orange Revolution" seemed remote.
Authorities put on a show of force, with busloads of riot police fanning out into nearby streets and courtyards and preventing people from approaching the city's main square on Monday.
Police had only a small and unobtrusive presence at the protest the previous night, when an estimated 10,000 people braved the freezing cold and snow to register outrage after Alexander Lukashenko, the president, was declared the winner of Sunday's elections.
Lukashenko asserted on Monday that his foes had failed to topple him in a foreign-backed "revolution".
International observers said the vote fell short of democratic standards; Europe's main human rights organisation said it was a farce and the United States called for a new election.
Lukashenko's main opponent refused to accept the outcome, calling the longtime leader an "illegal, illegitimate president".
However, the leverage of the international community seemed limited, and even many of the protesters appeared to have little appetite for a prolonged vigil and a possibly violent confrontation.
The election result, if it stands, would entrench the status of Belarus as one of the least independent of the former Soviet republics.
About 5000 protesters gathered in Oktyabrskaya Square in the capital's centre, about half the number that had come out on Sunday night for a protest whose size was extraordinary in a tightly controlled country where police have cracked down swiftly on unsanctioned opposition gatherings.
The diminished size of the crowd suggested the opposition was losing momentum, but Alexander Milinkevich, the main opposition presidential candidate and symbolic heart of the protests, called on the demonstrators to gird for a lengthy campaign.
He is demanding an election rerun.
Milinkevich is demanding an
"Our protest will be long and strong," he said.
"We will never recognise this election. It's not an election but an anti-constitutional seizure of power."
On the square, a 45-year-old woman who gave her name only as Irina said she was scared about the prospect of bloody police action, but "if Lukashenko stays in power, it will be even worse".
Milinkevich's appeal for a repeat vote was backed by the United States. White House spokesman Scott McClellan said the election was flawed by a "climate of fear", and hinted that penalties such as travel restrictions "are things we will look at".
"We support the call for a new election," McClellan said.
"The United States will continue to stand with the people of Belarus."
Vladimir Putin, the Russian president, on Monday congratulated Lukashenko and said the results would help strengthen the alliance of the two ex-Soviet nations.
The Kremlin said Putin sent a telegram to Lukashenko saying the vote "highlighted voters' trust in your course aimed at strengthening welfare of the Belarusian people".
The chief electoral official said on Monday that Lukashenko, who has ruled with an iron fist since 1994, won a "convincing victory" with 82.6% of the votes - a number Milinkevich called "monstrously inflated".
Lukashenko scorned the opposition, saying voters had shown "who's the boss" in Belarus.
Lukashenko: The much-talked-
about revolution has failed
"The revolution that was talked about so much ... has failed," he told a nationally televised news conference.
"You have seen our opposition, and if you are reasonable people you have been convinced that it's worthless," Lukashenko said.
Lukashenko called on the opposition to halt protests, saying the vote showed that he has overwhelming support.
"We must accept the decision of the people," he said.
He asserted that Sunday's protest leaders were in the pay of Western ambassadors and claimed there was no crackdown because the opposition is weak.
"Who was there to fight with? Nobody, understand? That's why we gave them the opportunity to show themselves, even though it was illegal."
Western countries have called Lukashenko Europe's last dictator and sharply criticised the election campaign.
The observer mission from the Organisation for Security and Cooperation in Europe said on Monday that the election did not meet standards for a free and fair vote.
Austria's Ursula Plassnik said the
opposition was intimidated
"Arbitrary use of state power and widespread detentions showed a disregard for the basic rights of freedom of assembly, association and expression," the OSCE mission said.
Ursula Plassnik, the Austrian foreign minister, whose country holds the EU's rotating presidency, said on Monday that the opposition "was systematically intimidated" during the campaign, and the EU threatened diplomatic and financial sanctions against Belarus' leaders.
In contrast, the Russian Foreign Ministry said the balloting took place "in accord with commonly accepted standards, and the legitimacy of their results does not provoke doubts".