German Foreign Minister Heiko Maas said "Such an act can not be left without definite consequences from the side of the European Union" and called for Pratasevich to be released.
The president of the European Council Charles Michel, who will host a summit of the 27 EU national leaders on Monday, also spoke out.
After hours grounded in Belarus, the flight was able to continue on its journey, landing in Vilnius at 1825 GMT, the airport's online arrivals board said.
Lithuanian President Gitanas Nauseda condemned Belarus's actions, demanding Protasevich be released and calling on for an worldwide response.
Roman Protasevich (26), the detained journalist worked for an online opposition news service NEXTA, a Telegram channel that broadcast footage of mass protests against Belarus President Alexander Lukashenko a year ago at a time when it was hard for foreign media to do so.
Ryanair, in a short statement, said the plane was told to divert to Minsk by Belarusian air traffic control because of a "potential security threat on-board".
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.
"We hold the government of Belarus responsible for the security of all passengers and the aircraft", European Union foreign policy chief Josep Borrell tweeted earlier.
Belarusian authorities said there were 123 passengers on the plane, which resumed its trip to Vilnius later Sunday.
Lithuanian President Gitanas Nauseda described the incident as "unprecedented" and blamed the Belarus regime for the "abhorrent action". An official said the Ryanair forced landing would also now be discussed.
Belta reported, citing airport official Maksim Kiyakov.
Mr Pratasevich, who had fled the country for Poland, faces charges that could carry a prison sentence of up to 15 years.
Belarus previous year designated Nexta an extremist organisation and has called for his extradition from Poland, where the channel's editorial team is based.
Nexta Live and its sister channel Nexta - with close to two million subscribers on Telegram - are prominent voices of the Belarus opposition and helped mobilise protesters.
"It is absolutely obvious that this is an operation by the special services to hijack an aircraft in order to detain activist and blogger Raman Pratasevich", she said in a statement.
The opposition says that Tikhanovskaya, who fled to neighbouring Lithuania after the election, was the true victor of last year's presidential vote.
It added that Protasevich said he was followed before boarding the flight.
"Then when the plane had entered Belarus airspace, the KGB officers initiated a fight with the Ryanair crew insisting there's an IED onboard", he said.
Raman Pratasevich, the former editor-in-chief of the most popular Telegram news channel in Belarus, was arrested in the Minsk airport on Sunday after the plane landed, according to the Minsk-based human rights center Viasna, which is not officially registered by the country's authorities.