MOSCOW—A Belarusian activist who avoided deportation by tearing up her passport last year was sentenced to 11 years in prison on a variety of charges, including plotting to oust the country’s leader, Alexander Lukashenko, effectively silencing the last main opposition voice still in the country.

A Minsk court found Maria Kalesnikava guilty of conspiring to seize power, creating an extremist group, and calling for actions that could damage national security, it said. Ms. Kalesnikava’s colleague Maxim Znak was sentenced to 10 years in a high-security prison.

Both dissidents served as members of the National Coordination Council, a panel that opposition leaders formed last year to help transfer power to a new government after a disputed presidential election. Mr. Lukashenko, Belarus’s longtime leader, claimed he had won the vote, but opponents and Western leaders said the vote was fraudulent and hundreds of thousands of Belarusians took to the streets to demand a change in government.

A security crackdown led to the detention of thousands of protesters; Viasna, a nongovernmental human rights group in Minsk reported that more than 650 political prisoners were being held as of Sept. 6. The opposition candidate Svetlana Tikhanovskaya, who ran in place of her jailed activist husband, fled the former Soviet republic for Lithuania almost immediately after the vote. She was joined by several others, many of them women, including Veronika Tsepkalo, whose husband, a former ambassador to the U.S., fled the country out of fear he would be arrested too.

Ms. Kalesnikava, the third of the three most-visible female leaders, opted to stay to help organize mass protests, defying government attempts to force her to join the others in exile.

In September last year, security forces took her to the frontier with Ukraine, but Ms. Kalesnikava ripped up her passport, preventing her from passing the border checkpoint. She was subsequently detained.

News of the sentencing after a monthlong, closed-door hearing drew swift condemnation from Western governments while Ms. Kalesnikava’s attorney said his client reacted calmly to the verdict, and said he would appeal to the country’s Supreme Court.

“I cannot say that she is surprised or somehow shocked by what she heard,” Uladzimir Pylchanka said. “She was not a bit upset and her mood didn’t change in any way after the verdict.

U.K. Foreign Secretary Dominic Raab tweeted that it “shows the Belarusian authorities continuing their assault on the defenders of democracy and freedom.”

He called on Mr. Lukashenko’s regime “to halt this repression and release all political detainees.”

In a statement, the European Union decried “the continuous blatant disrespect by the Minsk regime of the human rights and fundamental freedoms of the people of Belarus.”

The bloc, together with the U.S., has applied financial sanctions on Mr. Lukashenko’s regime to respond to the alleged election fraud, the escalation of serious human-rights violations and repression of civil society, in addition to other transgressions. On Monday, the EU said it would continue its efforts “to promote accountability for the brutal repression by the Belarusian authorities.”

Ms. Tikhanovskaya, who has campaigned vigorously for sanctions, struck a defiant tone on her Telegram channel Monday.

“The regime would love to see Max and Maria crushed and exhausted,” she wrote. “But we see our heroes strong and free inside.”

Write to Ann M. Simmons at ann.simmons@wsj.com