MINSK, BELARUS -- Prosecutors in Belarus opened a criminal investigation Thursday against opposition activists who set up a council to negotiate a democratic transition of power amid massive protests against official election results that extended the 26-year rule of the country's authoritarian leader.

Belarusian President Alexander Lukashenko, who has dismissed the protesters demanding his resignation as Western puppets, had threatened opposition leaders with criminal charges. Following up on his warning, prosecutors opened an inquiry against the new council's founders on charges of undermining national security.

The Belarusian Prosecutor General's office said the creation of the Coordination Council that met for the first time Wednesday violated the constitution.

"The creation and the activities of the Coordination Council are aimed at seizing power and inflicting damage to the national security," Prosecutor General Alexander Konyuk said.

The council members have rejected the accusations and insist their actions fully comply with Belarusian law. The United States on Thursday urged the authorities to engage in a dialogue with the opposition council and described the Aug. 9 presidential election that handed Lukashenko a sixth term as neither free nor fair.

The post-election protests continued in the capital of Minsk and other cities for the 12th straight day. About 1,000 protesters rallied on Minsk' central Independence Square, chanting "Go away!" to demand that the country's leader since 1994 leave office. .

"The Belarusians have changed," Olga Matusevich, a 29-year-old entrepreneur, said. "The protest will not end until Lukashenko steps down."

During the first four days of protests, police detained almost 7,000 people and injured hundreds with rubber bullets, stun grenades and clubs. At least three protesters died.

The crackdown fueled massive outrage and swelled protesters' ranks, forcing authorities to change tactics and stop breaking up crowds that grew to an unprecedented 200,000 on Sunday.

"I was born and grew up under Lukashenko and I heard nothing but threats," said 26-year-old biologist Maria Fando. "But it's impossible to scare the majority, and they won't be able to jail everyone."

After standing back for days, police again beefed up their presence on the streets of Minsk on Wednesday, blocking access to some government buildings and also deploying outside major factories where workers have been on strike since Monday while backing the anti-Lukashenko protests.

"They are again trying to scare us," Maxim Shukevich, 32, who works at the Minsk Tractor Plant, said. "The authorities are striking back. Our factory is being flooded with law enforcement agents."

The industrial action that has engulfed major factories across the country cast a tough challenge to Lukashenko, who had relied on blue-collar workers as his core support base.

In a bid to stop the strike from spreading, the Belarusian leader on Wednesday said that the participants would face dismissal and ordered law enforcement agencies to protect factory managers from opposition pressure.

Hundreds of state television employees have also gone on strike, shaking the government's control of the media.

The Belarusian leader warned members of the Coordination Council that they could face criminal responsibility for their attempt to create "parallel power structures."

The council has called for a new presidential election organized by newly formed election commissions, as well as for an investigation into the protest crackdown and compensation for victims of police violence.

The group's goals drew increasing support from governments in Europe and beyond this week. U.S. Secretary of State Mike Pompeo said Thursday that Washington remains "deeply concerned by serious flaws" in the the Aug. 9 election and strongly condemns the violence against protesters.

Pompeo said the vote in Belarus didn't meet the standards for free and fair elections. He urged officials there to "actively engage Belarusian society," including through the newly established opposition council, "in a way that reflects what the Belarusian people are demanding."

French President Emmanuel Macron said the European Union was ready to play a mediating role in Belarus if Belarusian protesters want that. EU leaders on Wednesday said they were preparing sanctions against Belarusian officials responsible for alleged election fraud and for police brutality against protest participants.

Macron, who discussed the situation in Belarus with Russian President Vladimir Putin this week, said Putin was "favourable" to the idea of mediation by the Organization for Security and Cooperation in Europe, the top trans-Atlantic security and rights group.

"We are ready to bring all our assistance and our mediation," the French president told reporters. "We insisted on the need to have a democratic, inclusive transition in Belarus that passes via dialogue."

The Belarusian opposition body consists of top associates of Sviatlana Tsikhanouskaya, who was Lukashenko's leading challenger in the Aug. 9 election, as well as rights activists and representatives of striking workers. It also includes Belarus' most famous author, Svetlana Alexievich, who won the 2015 Nobel Prize in literature.

The only former senior official on the council, Pavel Latushko, who headed a leading national theatre and was fired earlier this week for siding with protesters, said he wouldn't leave the country despite being threatened with arrest. The facade of his house in Minsk was splashed with red paint overnight.

Another opposition council member, Sergei Dylevsky, the leader of striking workers at the Minsk Tractor Plant, also dismissed the official accusations as "total nonsense."

"There is a sharp conflict between the people and the government, and we only represent a body that would try to mediate it," Dylevsky said. "We aren't aiming to overthrow the government and seize power, our goal is peaceful dialogue."

Prosecutors summoned Tsikhanouskaya's lawyer, Maxim Znak, who also sits on the council, to come for interrogation Friday as part of the probe.

Tsikhanouskaya, 37, a former English teacher who went to neighbouring Lithuania after the election in a move that her campaign aides said was made under pressure, met Thursday with Lithuanian Prime Minister Saulius Skvernelis, who promised to help "achieve free and fair elections in Belarus."

Facing Western criticism, Lukashenko, 65, has turned to Russia for help, saying that Putin promised him security assistance if Belarus needs it. The two nations have an agreement that contemplates close political, economic and military ties.

Asked about the EU's decisions, Putin spokesman Dmitry Peskov reaffirmed Moscow's warning against foreign meddling.

"We continue to convey our stance that we consider any foreign influence on the developments in Belarus unacceptable," he told reporters.

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Vladimir Isachenkov in Moscow, Matthew Lee in Washington, Angela Charlton in Paris and Liudas Dapkus in Vilnius, Lithuania contributed to this report.