Western countries have condemned Belarus for diverting a plane flying over its territory to arrest an opposition journalist.
EU leaders are due to discuss their response to what the union's executive called a "hijacking" and the US state department said was "a shocking act".
Belarus scrambled a fighter jet to force the plane - bound for Lithuania - to land, claiming a bomb threat.
Police took Roman Protasevich away when passengers disembarked.
The 26-year-old was aboard the Ryanair plane, which was flying from the Greek capital, Athens. The aircraft was due to land in Vilnius, but was still in Belarusian airspace when it was told to divert to Minsk, the capital of Belarus.
Witnesses said the activist was "super-scared" and told fellow passengers he would face the death penalty - Belarus is the only European country that still executes prisoners.
State media in Belarus said President Alexander Lukashenko had personally given the order for the move. The plane landed in Vilnius more than six hours after its scheduled arrival.
Since winning a disputed election last August, Mr Lukashenko, 66, who has ruled the country since 1994, has cracked down on dissenting voices. Many opposition figures have been arrested, while others fled into exile.
The incident drew sharp condemnation from across the European Union, with countries urging the immediate release of Mr Protasevich and a full investigation. The EU later summoned the Belarusian ambassador and informed him of the bloc's "firm condemnation".
Dozens of Belarusian officials, including President Lukashenko, are already under EU sanctions including travel bans and asset freezes, imposed in response to the repression on opponents.
Several countries, including the UK and Lithuania, have told flights to avoid Belarusian airspace. UK Transport Secretary Grant Shapps said he issued the instructions "in order to keep passengers safe".
How was the flight diverted?
Flight FR4978 turned east to Minsk shortly before it reached the Lithuanian border. Lithuanian officials, citing Ryanair, said 126 passengers boarded the plane in Greece.
In a statement, Irish carrier Ryanair said the crew had been "notified by Belarus (Air Traffic Control) of a potential security threat on board and were instructed to divert to the nearest airport, Minsk".
But Tadeusz Giczan - the editor of Nexta, the media outlet Mr Protasevich used to work for - tweeted that agents from Belarusian security service the KGB had boarded the plane and were the source of the bomb alert.
Franak Viacorka, a friend and associate of Mr Protasevich, told BBC Radio 4's Today programme that the journalist "already felt something bad" in Athens airport because he had seen that someone was following him.
Some passengers described seeing Mr Protasevich looking nervous as the incident unfolded. "He just turned to people and said he was facing the death penalty," Monika Simkiene told AFP news agency.
Another passenger told Reuters news agency that Mr Protasevich had opened an overhead locker after they were told of the diversion, pulled out a laptop and a phone and gave them to a female companion.
Mr Viacorka said the woman, who was Mr Protasevich's girlfriend and was arrested with him, was "not involved at all in anything, but they will be pursuing her because she's a close person to him". She has been named as Sofia Sapega, a Russian citizen.
"This was a case of state-sponsored hijacking... state-sponsored piracy," Ryanair chief executive Michael O'Leary told Irish Newstalk radio on Monday.
"It appears the intent of the authorities was to remove a journalist and his travelling companion... we believe there were some KGB agents offloaded at the airport as well," Mr O'Leary said.
Later on Monday, Lithuanian officials said five passengers who boarded the flight in Athens did not reach Vilnius. "The latest... is that 126 passengers left Athens and 121 landed in Vilnius. This does not include the crew of six," a spokeswoman for the Lithuanian prime minister told Reuters.
How will the West respond?
EU leaders are expected to discuss the incident at a summit in Brussels later on Monday. Nato ambassadors are due to meet on Tuesday.
A package of measures being worked on includes a ban on overflights of Belarus, a ban on entry to European airports by national carrier Belavia and a suspension of ground transport links, French officials say.
Ursula von der Leyen, head of the European Commission, said "the outrageous and illegal behaviour... will have consequences".
Lithuanian President Gitanas Nauseda urged the EU to impose fresh economic sanctions. He told the BBC this "could make a larger impact on the behaviour of the Belarusian regime".
The country has advised its citizens against travel to Belarus, and urged those already in the country to leave.
Irish Foreign Minister Simon Coveney said actions, not words, and sanctions with "a real edge" were needed.
"We cannot allow this incident to pass on the basis of warnings or strong press releases," he told state broadcaster RTE.
UK Foreign Secretary Dominic Raab said Mr Protasevich had been arrested "on the basis of a ruse" and called for his immediate release.
Why would Mr Protasevich have been targeted?
Mr Protasevich is a former editor of Nexta, a media operation with a Telegram channel. He left Belarus in 2019 to live in exile in Lithuania. From there he covered the events of the 2020 presidential election, after which he was charged with terrorism and inciting riots.
Nexta played a key role for the opposition during the vote, which was won by Mr Lukashenko and is widely regarded as rigged. It has continued to do so in its aftermath, particularly with the government imposing news blackouts.
But he first attracted the attention of the authorities as a teenager, when he was expelled from school after taking part in a protest in 2011.
Mr Protasevich now works for a different Telegram channel, Belamova. He stepped in to write for it after blogger Igor Losik was arrested by the Belarusian authorities in June last year.
He was in Athens to attend an economic conference along with Svetlana Tikhanovskaya, the opposition leader who claimed victory in the election. She too now lives in exile in Lithuania.
Belarus: The basics
Where is Belarus? It has its ally Russia to the east and Ukraine to the south. To the north and west lie EU and Nato members Latvia, Lithuania and Poland.
Why does it matter? Like Ukraine, this nation of 9.5 million is caught in rivalry between the West and Russia. President Lukashenko has been nicknamed "Europe's last dictator" - he has been in power for 27 years.
What's going on there? There is a huge opposition movement demanding new, democratic leadership and economic reform. The opposition movement and Western governments say Mr Lukashenko rigged the 9 August election. Officially he won by a landslide. A huge police crackdown has curbed street protests and sent opposition leaders to prison or into exile.
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