KYIV, Ukraine (AP) — Authorities intensified their crackdown on dissent in Belarus ahead of planned protests against the nation's authoritarian president and detained dozens of people Wednesday, according to human rights activists.

The opposition is planning demonstrations Thursday to mark Freedom Day, the anniversary of the country's 1918 declaration of independence.

Protests have rattled Belarus since the summer, when official results from an August presidential election gave President Alexander Lukashenko a sixth term in office by a landslide. The opposition and some poll workers have said the election was rigged.

The president's top challenger, Sviatlana Tsikhanouskaya, urged Belarusians to launch a new wave of anti-Lukashenko rallies starting Thursday.

Authorities have banned any demonstrations. The chief of Belarus' security agency, which still goes by the Soviet-era name KGB, warned against “attempts to destabilize the situation in the country.”

Police detained at least 42 people in 10 Belarusian cities on Wednesday, according to the Viasna human rights center.

Earlier this week, police also arrested Mikalay Kazlou, the leader of the opposition United Civic Party, and Andżelika Borys, head of the Union of Poles of Belarus.

Borys was sentenced to 15 days in jail on charges of organizing an unsanctioned meeting, which her group dismissed as “an act of intimidation of the Polish minority.”

In another move to stifle dissent, Belarus' Justice Ministry announced Wednesday that it had stripped a lawyer who has defended opposition activists and independent journalists, Siarhej Zikratski, of his license to practice.

Lukashenko’s government unleashed a sweeping crackdown on the post-election protests that erupted last year, the biggest of which attracted up to 200,000 people. Human rights activists say more than 33,000 people have been detained since the demonstrations began, with thousands beaten.

The United States and the European Union have responded to the election and the crackdown by introducing sanctions against Belarusian officials.

Amnesty International expressed concern about authorities attempting to squelch Freedom Day protests and urged foreign diplomats in Belarus to monitor the events for possible violence by police.

"Over the past eight months, the Belarusian government has presided over a human rights crisis marked by mass arbitrary arrests, imprisonment of journalists, unlawful use of force during protests, and torture and killing of peaceful protesters,” Marie Struthers, Amnesty International's Eastern Europe and Central Asia director, said in a statement. "The message from the top in Belarus is clear: any exercise of the right to freedom of peaceful assembly or expression will be ruthlessly crushed. The world cannot stand by in the face of these threats.”