KYIV, Ukraine (AP) — Belarus’ authorities on Monday handed a jail sentence to a factory strike organizer and detained a leading opposition activist, part of a methodical effort to stifle weeks of protests against the country’s authoritarian leader after an election the opposition says was rigged.

President Alexander Lukashenko, who has ruled the 9.5-million nation with an iron fist for 26 years, has dismissed the protesters as Western puppets and rejected the European Union’s offers of mediation. After a ferocious crackdown on demonstrators in the first days after the Aug. 9 presidential vote that caused international outrage, his government has avoided large-scale violence against demonstrators and switched to threats and the selective jailing of activists to stem the protests.

Anatoly Bokun, who leads the strike committee at Belaruskali, a huge potash factory in Soligorsk, was detained by police Monday and handed a 15-day jail sentence on charges of organizing an unsanctioned protest. The factory, which accounts for a fifth of the world’s potash fertilizer output, is the nation’s top cash earner.

The Belaruskali strike committee spokesman, Gleb Sandras, said authorities had managed to halt a strike at the factory that began two weeks ago and all its potash mines are now working. He said agents of Belarus’ State Security Committee, which still goes by the Soviet-era name KGB, had pressured workers to end the strike.

“KGB agents have inundated the factory, tracking down the most active workers and using various means of pressure,” Sandras told The Associated Press. “The authorities have powerful economic instruments. They are blackmailing workers with mass dismissals.”

Strikes at Belaruskali and many other leading industrial plants have cast an unprecedented challenge to Lukashenko, who has kept the bulk of the economy in state hands and relied on blue-collar workers as his main support base.

Belarus Deputy Prime Minister Yuri Nazarov acknowledged Monday that the strikes posed a problem, but said all major industrial plants have resumed normal operations.

Bokun’s detention follows the arrests of strike leaders at two other major industrial plants in Minsk last week. The organizer of a strike at the Grodno Azot, a major producer of nitrogen fertilizers, fled to neighboring Poland to escape detention.

Seeking to stem the protests, Belarusian prosecutors have opened a criminal probe against the opposition Coordination Council created to negotiate a transition of power, accusing its members of undermining the country’s security.

Last week, Belarusian courts handed 10-day jail sentences to two council members and summoned several others for questioning, including Svetlana Alexievich, who won the 2015 Nobel Prize in literature. Another leading council member, Lilia Vlasova, was detained by police on Monday.

“This is the government’s response to our peaceful actions and offers of dialogue,” council member Maria Kolesnikova told the AP. “It means that protests will grow.”

Belarusian authorities on Monday also denied entry to Tadeusz Kondrusiewicz, the 74-year-old archbishop of Minsk and Mohilev. Yury Sanko, a spokesman for the Catholic Church in Belarus, said the Belarusian border guards kept Kondrusiewicz waiting for four hours on the border before turning him back to Poland.

Last week, Kondrusiewicz strongly criticized Belarusian police, who locked the doors of a Catholic church in the Belarusian capital of Minsk where several dozen protesters found refuge as police were dispersing a protest.

Both the U.S. and the EU have criticized the Aug. 9 election that extended Lukashenko’s rule as neither free nor fair and urged Belarusian authorities to talk with the opposition — calls that the 66-year-old leader has rejected.

EU foreign ministers agreed last week to prepare a sanctions list of up to 20 senior Belarus officials suspected of election fraud and the crackdown on protesters. On Monday, the EU’s Baltic members — Lithuania, Latvia and Estonia — slapped their own travel sanctions on 30 top Belarusian officials, including Lukashenko.

Belarus’ Foreign Ministry spokesman, Anatoly Glaz, warned that Minsk would retaliate. Last week, Lukashenko threatened to respond to the sanctions by redirecting the flow of Belarusian imports via Lithuanian ports and blocking the transit of European cargo across Belarusian territory.

The detention of hundreds of demonstrators last week didn’t deter the opposition from mounting another massive rally on Sunday, which saw an estimated 100,000 flood the streets of Minsk amid a heavy police presence.

German Chancellor Angela Merkel’s spokesman, Steffen Seibert, hailed the protesters’ courage and urged Lukashenko on Monday to “recognize the reality in the country — there needs to be an open dialogue between the leadership, opposition forces and all of Belarusian society to bring about a peaceful solution.”

In a bid to win time and to assuage simmering discontent, Lukashenko touted the prospect of a vague constitutional reform that could see a new presidential election at an unspecified time. On Monday, he bluntly dismissed the opposition’s push for restoring the country’s earlier constitution, which envisaged broad parliamentary powers.

A fierce clampdown on peaceful demonstrators after the vote left nearly 7,000 people detained, hundreds injured by police rubber bullets, stun grenades and beatings and at least three protesters dead. Police then let the demonstrations go unhindered for the next two weeks, but last week again cranked up the pressure on demonstrators and began dispersing rallies.

Over the weekend, the Belarusian government also cracked down hard on the news media, deporting some foreign journalists from the country and revoking the accreditation of many Belarusian journalists. Two Moscow-based Associated Press journalists covering the protests in Belarus were deported to Russia on Saturday. In addition, the AP’s Belarusian journalists were told by the government that their press credentials had been revoked.

The Belarusian Association of Journalists said that accreditation rights were also taken away from 17 Belarusians working for several other media, including Germany’s ARD television, the BBC, Reuters and AFP. The U.S.-funded radio Free Europe/Radio Liberty said five of its journalists lost their accreditation.

The U.S. and the EU officials have strongly condemned the media crackdown.

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Vladimir Isachenkov in Moscow, Geir Moulson in Berlin and Liudas Dapkus in Vilnius, Lithuania contributed to this report.

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Follow AP’s coverage of Belarus at https://www.apnews.com/Belarus