Leading Belarus opposition politician Nikolai Statkevich was jailed on Monday amid a crackdown on protesters who rallied against President Alexander Lukashenko's re-election bid.
Statkevich was handed a 15-day jail term after taking part in Sunday's protest in Minsk, according to Belarusian human rights group Viasna.
In 2011, Statkevich was sentenced to six years in prison for involvement in protests after the 2010 presidential election - where he had unsuccessfully run as a candidate - and is barred from running again
Amnesty International reported in 2012 that he was facing 10 days in a punishment cell "for refusing to sign a confession".
According to Viasna, other 50 opposition activists were arrested by police last week while they were gathering signatures to allow opponents to participate in the presidential election, bringing arrests to more than a hundred in May.
People arrested included supporters of blogger Sergei Tikhanovsky, who was banned from running in the elections, too.
Viasna said the interior ministry told them that a criminal investigation had been opened into an alleged act of violence against police officers, in which Tsikhanouski and other detainees were allegedly involved.
With regards to the upcoming election, Viasna has in particular criticised the formation of the election commissions, the bodies overseeing the voting process.
The human rights group condemned the lack of "legal guarantees for the representation in the election commissions of all political entities participating in the election results in an arbitrary and discriminatory approach to opposition parties and groups".
Viasna's criticism is echoed by the EU, which condemned the crackdown on "peaceful" protesters and said to be "worried" about Belarus' Central Election Commission's decision to bar "prominent opposition figures" from registering for the election.
Lukashenko has been Belarus' president since the office was established, in 1994, and is seeking a sixth term in the presidential vote scheduled for August 9.
During legislative elections last year, the opposition did not win any seats in parliament.
The electoral process was criticised by a joint observation mission formed by the Organization for Security and Cooperation in Europe (OSCE) and the Council of Europe.
"Fundamental freedoms were disregarded and the integrity of the election process was not adequately safeguarded in the early parliamentary elections in Belarus even if the number of registered candidates increased," it said.
“These elections have demonstrated an overall lack of respect for democratic commitments,” special Co-ordinator and leader of the OSCE short-term observer mission Margareta Cederfelt said.
“In a country in which the power and independence of parliament are limited and fundamental freedoms are restricted for both voters and candidates, parliamentary elections are in danger of becoming a formality.”