A spokesman said President Alexander Lukashenko personally ordered that a MiG-29 fighter jet accompany the plane - which was travelling from Athens to Vilnius in Lithuania - to an airport in Minsk.
He said he could not be sure if Protasevich's companion, who took the laptop and phone, had also been detained in Minsk, but there appeared to be more empty seats on the final Minsk-Vilnius journey than when it took off from Athens.
Protasevich, 26, worked for an online opposition news service NEXTA, a Telegram channel that broadcast footage of mass protests against Mr Lukashenko previous year at a time when it was hard for foreign media to do so.
The other passengers looked tired on arrival in Vilnius, with Mantas saying they had spent hours in queues for luggage and passport checks in Minsk before being allowed into a lounge to await takeoff.
Christophe Deloire, the head of Reporters Without Borders, called the moves against Tut.by "a new attempt to silence the most well-known independent media in Belarus", and called on the United Nations to act to ensure freedom of information.
Pratasevich, who had fled the country for Poland, faces charges that could carry a prison sentence of up to 15 years.
The Ryanair commercial flight was directed to land "by force", the president said in an emailed statement.
Lithuania and Latvia have called for worldwide flights not to use Belarusian airspace.
"This shocking act perpetrated by the Lukashenka regime endangered the lives of more than 120 passengers, including USA citizens", he said in a statement, using an alternative spelling of the Belarusian leader's name.
The journalist had his head in his hands and was shaking when he realised the flight was headed for Minsk, Lithuania's Delfi news outlet said, quoting a passenger.
She stood against Mr Lukashenko in presidential elections, which she says she would have won had they been free and fair.
"I call on North Atlantic Treaty Organisation and European Union allies to immediately react to the threat posed to global civil aviation by the Belarus regime".
The forced landing came before a two-day meeting in Brussels of European Union leaders, who will discuss consequences and possible sanctions, European Union spokesman Barend Leyts said. "The worldwide community must take immediate steps that this does not repeat", Mr Nauseda said.
European Council president Charles Michel said in a statement: "I call on Belarus authorities to immediately release the detained passenger and to fully guarantee his rights. Consequences and possible sanctions will be discussed at this occasion".
Antony Blinken, the US secretary of state, sharply criticized the government of Belarus on Twitter on Sunday for its detention of Protasevich.
Extremely worrying reports of a Ryanair plane forced to land in Belarus to detain an opposition blogger. "This is a serious and unsafe incident, which requires worldwide investigation".
Polish Prime Minister Mateusz Morawiecki branded as an "act of state terrorism" the arrest of Protasevich after his flight was forced to make an emergency landing.
"I condemn in the strongest terms the detention of Roman Protasevich by Belarusian authorities, after a Ryanair passenger aircraft was hijacked".
The Foreign Ministry of Greece also condemned the forced landing, in what it called a "state hijacking".
US Secretary of State Antony Blinken called the incident "shocking" and accused Lukashenka's government of endangering the lives of those aboard the aircraft, some of them Americans.
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