Factory workers chanted slogans, students and pensioners took to the streets, and police used force to detain people on Monday.
As Belarusians answered an opposition call for nationwide strikes to force President Alexander Lukashenko to quit.
Lukashenko defied an ultimatum to surrender power by midnight, challenging his opponents to make good on a threat to paralyze the country,
Nearly three months after his disputed election victory unleashed mass protests...often met with force like this, on Sunday, when police fired stun grenades and detained scores of people at protests in Minsk and elsewhere.
Exiled opposition candidate Sviatlana Tsikhanouskaya stepped up her calls for the strikes to go ahead.
If sustained, the strikes could open a new phase in the crisis, testing whether the opposition has the mass support it needs to bring the country of 9.5 million people to a halt.
Tsikhanouskaya has urged Belarusians to block roads, shut down workplaces, stop using government shops and services and withdraw all money from their bank accounts.
In response, Lukashenko has scoffed: "Who will feed the kids?"
On Monday, local media reported groups of strikers at many major state-controlled enterprises.
However, the prime minister's spokeswoman said all the country's major industrial companies were working normally.