The detainee is said to be Roman Protasevich, who, like other prominent opposition figures, is living in exile due to the harsh rule of President Alexander Lukashenko.
Lithuanian President Gitanas Nauseda demanded on Sunday afternoon that Belarus release Protasevich, and Lithuania also calls on NATO and the EU to act.
"I demand that Roman Protasevich be released immediately," Gitanas Nauseda wrote on Twitter.
The plane must have been examined
Various information about why the plane was forced to land abounds.
According to some sources, it must have happened after a bomb threat.
According to other sources, fighter jets and helicopters have forced it down.
None of this information is confirmed.
Reuters reports that the plane was investigated after landing in Belarus and that no bomb was found on board.
According to Flightradar24, it was a Ryanair plane of the type Boeing 737 which took off from Athens on Sunday morning and was on its way to the Lithuanian capital Vilnius, when the plane was instead diverted to Minsk.
About 170 passengers from 12 different countries are said to have been on board the plane, according to Reuters.
Information: Lukashenko gave orders
The Russian state news agency Tass writes that the plane - according to Minsk airport - was exposed to a bomb threat.
But according to the radio channel Komersant, the plane must have been diverted and landed immediately at the personal request of Belarus' President Alexander Lukashenko.
The airport in Vilnius informs that the plane is delayed and is expected to land later on Sunday.
"Staged by the intelligence service"
"It is quite clear that this is an operation staged by the intelligence service, to gain access to the plane and imprison the activist and blogger Roman Protasevich," writes Svetlana Tichanovskaya, opposition leader in Belarus, living in exile in Lithuania, on Telegram.
Both Tikhanovskaya and Nexta state that 26-year-old Protasevich risks the death penalty in his home country. Nexta has played a key role in organizing the opposition during the widespread protests against dictator Alexander Lukashenko since the presidential election last summer.