By Ann M. Simmons

MOSCOW -- Belarusian President Alexander Lukashenko scrambled a jet fighter to force a Ryanair commercial aircraft to land in Minsk, where a Belarusian journalist and opposition activist was detained. The Sunday incident sparked an international outcry and raised questions over the legality of the plane's grounding.

The aircraft was flying from Athens to Vilnius, Lithuania, when it was diverted to land in the Belarusian capital.

A spokesman for Ryanair said the flight's crew were notified by Belarus air-traffic control of a potential security threat on board and were instructed to divert to the nearest airport, which was Minsk.

A statement on Mr. Lukashenko's official Telegram messenger app said Belarusian authorities received information that a passenger plane over the country's airspace possibly had explosives on board and Mr. Lukashenko ordered a MiG-29 jet fighter to escort the aircraft to land in Minsk.

Authorities didn't provide details about the source of the information. Neither Mr. Lukashenko's office nor Belarus's Investigative Committee or emergency-services agency immediately responded to requests for comment clarifying the incident, including the chronology of events and exactly what occurred.

Belarusian opposition leaders said Roman Protasevich, a prominent activist and former editor in chief of the antigovernment NEXTA Telegram channel, was detained in Minsk.

Under Mr. Protasevich's leadership, the channel, which has around 1.5 million subscribers in Belarus, has become of the most reliable sources of independent information and live news videos, including about the wave of antigovernment demonstrations that swept the country last year. The protests followed a presidential vote that Mr. Lukashenko claimed he won but that opponents and Western leaders say was fraudulent.

In a statement Sunday night, U.S. Secretary of State Antony Blinken condemned the action and called for Mr. Pratasevich's immediate release.

"This shocking act perpetrated by the Lukashenka regime endangered the lives of more than 120 passengers, including U.S. citizens," Mr. Blinken said. "Initial reports suggesting the involvement of the Belarusian security services and the use of Belarusian military aircraft to escort the plane are deeply concerning and require full investigation."

Mr. Blinken added that the U.S. was closely coordinating its response with "our partners, including the EU and Lithuanian and Greek officials."

Branded an extremist by Mr. Lukashenko's regime, Mr. Protasevich was recently living in Poland and Lithuania, according to information published by his supporters. According to Belarus state television, law-enforcement officers had put him on a wanted list for allegedly organizing mass riots.

The grounding of the Ryanair plane prompted widespread international condemnation. Industry officials said they couldn't immediately recall another example of a commercial passenger jet flying far from any war zone being forced to land.

The head of the European Commission, Ursula von der Leyen, tweeted that "the outrageous and illegal behavior of the regime in Belarus will have consequences," and those responsible for what she called "the Ryanair hijacking" must be sanctioned, she said. Barend Leyts, spokesman for the European Council president, said the bloc would discuss possible sanctions at a scheduled meeting on Monday.

NATO Secretary-General Jens Stoltenberg said on Twitter: "This is a serious & dangerous incident which requires international investigation."

The International Civil Aviation Organization expressed concern that the forced landing of the aircraft could be in violation of the Chicago Convention that coordinates international air travel.

An adviser to Poland's prime minister, Mateusz Morawiecki, said the leader would push for an EU ban on Belarusian airspace and for no flights of Belarus's national carrier, Belavia, to enter Europe.

Belarusian opposition leader Svetlana Tikhanovskaya, who was Mr. Lukashenko's main rival in last August's presidential vote, chided the Belarusian leader for creating what she called an international scandal and risking passengers' lives for the sake of seeking revenge against a single individual.

Ms. Tikhanovskaya, who is widely believed to have won the presidential election, was forced to flee to Lithuania as Mr. Lukashenko unleashed a wave of violent reprisals against his opponents.

For six months, hundreds of thousands of Belarusians from across the social spectrum braved police beating and strong-arm tactics to join weekly protests demanding that Mr. Lukashenko, in power since 1994, step down. Several prominent opposition figures have since left the former Soviet state, some in fear of arrest, others by force.

On Facebook, Lithuanian President Gitanas Nauseda demanded Mr. Protasevich's release and appealed to NATO and EU allies "to respond to the threat of the Belarusian regime to international civil aviation.... so that such incidents can't happen again."

Last year, the U.S. and EU imposed sanctions against Belarus officials, part of a coordinated effort by Western allies to censure the regime over accusations of political repression and rigging elections.

Investigators found no explosive devices onboard the Ryanair plane, according Belarusian authorities. The country's investigative committee said in a telegram statement that a criminal case had been opened into the affair based on statutes to do with "knowingly falsely reporting danger."

At 8:48 p.m. airport authorities reported the aircraft had departed Minsk for Vilnius.

--Benjamin Katz in London, James Marson in Brussels and Sabrina Siddiqui in Washington contributed to this article.

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

(END) Dow Jones Newswires

05-23-21 1934ET