Vital Rymasheuski was released overnight on December 31-January 1 on condition that he not leave the city.
A colleague from Rymasheuski's Belarusian Christian Democracy party confirmed the news.
Denis Sadovski of the party said that Rymasheuski was expected to be questioned by the State Security Committee again on January 4.
His release was reportedly intended to allow him an opportunity "to write an explanatory note to [President Alyaksandr] Lukashenka," according to RFE/RL's Belarus Service.
Rymasheuski planned to hold a press conference on January 4 but that event was canceled, reportedly because such public statements could contravene the terms of his release.
Rymasheuski and four other candidates who ran against incumbent President Lukashenka have been charged in connection with the demonstrations that broke out in Minsk after the December 19 vote that officials say gave Lukashenka a fourth term.
They could face up to 15 years in prison if convicted.
More than 600 people were detained in the bloody crackdown on postelection protesters.
OSCE Working To Continue Belarus Work
Meanwhile, the OSCE chairperson in office, Lithuanian Foreign Minister Audronius Azubalis, says that chairmanship has begun consultations to find a solution that would enable the OSCE to continue its work in Belarus.
Azubalis said he "deeply regrets" that Belarusian authorities did not extend the mandate of the OSCE Office in Minsk.
Azubalis said the OSCE "started informal consultations to find an agreement acceptable to all."
U.S. State Department spokesman Philip Crowley questioned Minsk's decision to shutter the OSCE office in the wake of the December 19 vote.
"The United States deeply regrets the government of Belarus's decision to terminate the mission of the OSCE office in Minsk," Crowley said. "It was founded to assist the Belarusian government in institution-building, promoting the rule of law, and encouraging outreach to civil society, and the mandate of that mission is not completed, as the OSCE's critical assessment of the presidential elections indicates."
Crowley called the expulsion "a step backwards in the development of democratic government and respect for human rights in Belarus."
The OSCE has maintained a presence in Minsk since 1998.
Its mandate must be renewed annually by the 56 OSCE participating states, and expired on December 31 after Belarusian authorities refused to extend it.
based on RFE/RL and agency reports