"He was released slightly earlier than we expected," she said.
Kasparov, who has spoken of his intention to run in presidential elections scheduled for March 2, 2008, was expected to be released at 4:30 p.m. Moscow time (1:30 p.m. GMT) but was released at 4:00 p.m. (1:00 p.m. GMT).
The Russian presidential campaign kicked off on November 28.
Chess grandmaster Kasparov was arrested in central Moscow on Saturday while leading a march organized by the opposition group The Other Russia, which comprises the People's Patriotic Union, led by former premier Mikhail Kasyanov, the banned National Bolshevik Party, headed by writer Eduard Limonov, as well as Kasparov's United Civil Front.
Human rights advocates in Russia and abroad have criticized the Kremlin for tightening its grip on democracy and human rights ever since Vladimir Putin became president in 2000 and condemned Kasparov's detention. MA Amnesty International claimed that his arrest was planned before the march.
President Putin attacked opposition groups during a televised speech on Thursday, saying that they were seeking to "bring back the time of humiliation, dependence and disintegration."