MOSCOW — On the final day of what Belarus's opposition has called the "People's Ultimatum" — a demand that President Alexander Lukashenko resign or face a nationwide strike — tens of thousands of protesters marched in Minsk on Sunday before police cracked down with stun grenades.

The ultimatum was issued two weeks ago by the main opposition candidate in the Aug. 9 presidential election in an attempt to gain an edge in the stalemate with Lukashenko's government since the disputed vote. Lukashenko, who has ruled Belarus since 1994, claimed victory with about 80 percent of the vote — a result that many Western leaders have agreed was the product of electoral fraud.

Svetlana Tikhanovskaya, the main challenger, demanded that Lukashenko resign by Monday to make way for a new election, end police violence against protesters and release all political prisoners.

“Today at 23:59, the term of the People’s Ultimatum will expire, and if the demands are not met, the Belarusians will start a national strike,” Tikhanovskaya, who has been in self-exile in Lithuania since August, said in a statement Sunday.

Lukashenko has given no indication that he is amenable to Tikhanovskaya’s terms, and it’s unclear whether Belarusians will heed her call for a strike. While many workers at major state-owned factories and other enterprises went on strike in the first week after the election, those stoppages were short-lived because of threats that those employees would lose their jobs permanently.

But authorities prepared for mass demonstrations on Sunday. They closed 12 subway stations in Minsk and shut down mobile Internet services.

Riot police did not initially disperse the large crowd in the capital, which was estimated to rival the more than 200,000 who took to the streets on the first weekend of post-election protests. The crackdown came after nightfall. Videos on social media showed police firing on protesters, followed by what appeared to be stun-grenade blasts.

Security forces used tear gas in the western town of Lida, according to the Russian news agency RIA, which quoted the local interior ministry.

“It looks like wartime in my country,” Hanna Liubakova, a Minsk-based journalist, said on Twitter.

Secretary of State Mike Pompeo spoke with Lukashenko by phone on Saturday — the first conversation between the two since the election. Pompeo “called for the full release and immediate departure from Belarus of wrongfully detained U.S. citizen Vitali Shkliarov and re­affirmed U.S. support for the democratic aspirations of the people of Belarus,” a State Department spokesperson said in a statement to Reuters.

Shkliarov, who has advised presidential candidates in the United States, Russia and Ukraine, was released under house arrest last week.

Lukashenko told Pompeo that Belarus and Russia were ready to respond jointly to external threats, according to Russian news agencies quoting Belarusian state media. Lukashenko has turned east for a lifeline from Russian President Vladimir Putin. In late August, Putin said on Russian state television that a reserve military contingent was ready to intervene on Lukashenko’s behalf if protests got “out of control,” citing looting as an example.

Lukashenko has repeatedly accused Western countries of meddling in Belarusian affairs, alleging that foreign actors instigated the protests to force his ouster. The United States, the European Union, Britain and Canada have imposed sanctions against several high-ranking officials in Belarus accused of fraud and human rights abuses in the aftermath of the election.

The European Parliament on Thursday awarded the Sakharov Prize, its highest human rights recognition, to the Belarusian opposition movement.