Belarusian authorities have detained President Alexander Lukashenko's main rival in August's election.
Viktor Babaryko, a 56-year-old former banker, was detained on suspicion of financial crimes amid increased harassment of opposition figures.
Hundreds took to the streets in the capital Minsk to protest against recent arrests of government critics.
President Lukashenko, who has ruled the country since 1994, is seeking a sixth term in office.
Mr Babaryko's 30-year-old son Eduard, who is running his election campaign, was also arrested on suspicion of tax evasion.
"Babaryko is detained because he was the organiser and leader of illegal activities, (and) tried to influence witness testimony," Ivan Tertel, the head of the state control committee, told the AFP news agency.
He accused Mr Babaryko of colluding with "puppeteers" from Moscow.
Mr Babaryko and his son were on their way to submit signatures required to register as candidates when they stopped answering phone calls, his campaign website said.
"Viktor and Eduard are 'giving testimony,' which is an investigative action without the presence of lawyers," his Twitter account wrote.
"Viktor and Eduard's lawyers were not allowed inside, citing what they called 'drills' inside the building," it added.
Mr Babaryko was once head of Belgazprombank, a Belarus subsidiary of Russian energy giant Gazprom, before entering politics.
His arrest comes at a time of increased pressure on government critics ahead of the August election. Prominent opposition politician Mikola Statkevich was also recently jailed.
European Commission President Ursula von der Leyen said "all arbitrarily detained" politicians and activists should be released.