Leading Hong Kong democracy activist Joshua Wong was arrested Friday, his Demosisto party said, a day ahead of a planned rally in the city that has been banned by the police.
His Demosisto colleague, Agnes Chow, was arrested at her home in Tai Po just hours later, the party said.
It was not immediately clear what the charges were.
Mr Wong, the face of Hong Kong's push for full democracy during protests in 2014 that paralysed parts of the city for 79 days, was released from jail in June after serving a five-week term for contempt of court.
"He was suddenly pushed into a private car on the street," Demosisto said on its official Twitter account.
"He has now been escorted to the police headquarters in Wan Chai," it said, adding its lawyers were working on the case.
Saturday marks the fifth anniversary of Beijing's rejection of a call for universal suffrage in the city, a decision that sparked the 79-day Umbrella Movement led by mainly young protesters including Mr Wong.
Permission for a mass rally this weekend was denied on security grounds, raising the likelihood of another weekend of clashes between police and protesters, who will likely come out in defiance of the ban.
In a letter to the rally organisers the Civil Human Rights Front (CHRF), police said they feared some participants would commit "violent and destructive acts".
The protests ignited when the city's Beijing-backed government tried to pass a bill allowing extraditions to mainland China.
But they have evolved into a wider call for greater democracy and an investigation into allegations of police brutality.
