MINSK, Belarus — A Belarusian human rights group says police detained dozens of demonstrators in the capital and the city of Borisov on Wednesday as protests against the exclusion of two opposition candidates from the presidential ballot roiled the country.

In Minsk, thousands of people stood in a 3-kilometer-long (2-mile-long) line outside the national elections commission to sign complaints about the removal of Viktor Babariko and Valery Tsepalko from the ballot for the Aug. 9 election.

The two candidates were seen as the strongest challengers to authoritarian President Alexander Lukashenko, who is seeking a sixth term. Police closed off the center of Minsk, and the human rights group Vesna said at least 20 people were arrested. Another 15 demonstrators were detained in the city of Borisov, the group said.

Thousands of people also took to the streets of Minsk and other cities in protest on Tuesday, and police said 250 were detained.

The central election commission of Belarus allowed five presidential candidates to be named on the ballot, denying spots to Tsepkalo, founder of a successful high-technology park and a former ambassador to the United States, and former banker Babariko.

The decision eliminated any serious competition for Lukashenko, who is seeking reelection after already spending a quarter-century in power.

Lukashenko has ruled Belarus since 1994, stifling opposition and independent news media. He accused protesters of plotting a revolution and promised to protect the country from one.

"We will be defending the country with any lawful means. We will not surrender our country to anyone," he said.

Amnesty International on Wednesday condemned mass detentions as provoking violence and violating protesters' rights in Belarus.

"The police sought to disperse peaceful gatherings, with excessive and unnecessary use of force and in many cases deploying police officers in plainclothes. This provoked violent responses from some protesters who tried to prevent others being arrested and beaten," Aisha Jung, Amnesty International's senior campaigner for Belarus, said in a statement.

"However, according to eyewitnesses and widely available video footage, the gatherings remained largely peaceful, and many of those arrested were peaceful protesters," Jung said.

Belarusian Interior Ministry spokeswoman Olga Chemodanova said Wednesday that police demonstrated "self-restraint and high professionalism" despite "the aggression of certain individuals."

The Office for Democratic Institutions and Human Rights of the Organization for Security and Cooperation in Europe said in a statement that it won't be deploying an election observation mission to Belarus due to a "lack of invitation" from the country's government.