Belarusians chanting “Step down!” filled the centre of the capital Minsk on Sunday in the biggest protest so far against what they said was the fraudulent re-election a week ago of longtime president Alexander Lukashenko.

Russia said it would offer Lukashenko military help if necessary, but there was no visible police presence at the protest, which attracted around 200,000 people, a Reuters reporter estimated. At least two protesters have died and thousands have been detained in a crackdown since the vote.

The atmosphere at the rallies, which wrapped up late into the evening, was...