Belarus' President Alexander Lukashenko says 11 people were killed and hundreds injured in an explosion in the metro system of the capital, Minsk.
He called for a moment of silence to honour those killed in the blast, which struck a metro station close to his main office and residence.
The cause of the explosion, at 1755 local time (1555 GMT), is not known.
But in televised remarks, Mr Lukashenko said it was aimed at undermining "peace and stability".
And he hinted at foreign involvement, linking the explosion to a blast at a concert in 2008 in which about 50 people were injured.
"These are perhaps links in a single chain. We must find out who gained by undermining peace and stability in the country, who stands behind this," Mr Lukashenko said.
"I do not rule out that this [blast] was a gift from abroad."
Witnesses said there was a flash and a bang as passengers got off a train which had just pulled into the Oktyabrskaya metro station in the city centre.
One said at least part of the station's ceiling had collapsed after the explosion.
An eyewitness, Maskim Lew, told Ekho Moskvy radio: "There are a lot of special forces troops, a lot of ambulances and firemen.
"Some people are being treated on the spot, some are being taken away, some - those who are conscious and in a more or less normal state - are being helped into ambulances."
The metro was all cordoned off, he said.
A police spokesman told reporters Minsk had been placed on "heightened alert".
Exits from the station lead both to Mr Lukashenko's main office and his residence, as well as the country's powerful security council.
Mr Lukashenko, who has ruled Belarus since 1994, claimed victory in a presidential election last December, but international observers condemned the vote.
Immediately afterwards, dozens of opposition figures - activists and presidential campaigners - were rounded up and arrested. Some are still in jail.
Tensions are rising in the former Soviet republic, says the BBC's David Stern in Kiev. As well as the political tensions, Belarus has also suffered economic difficulties since the beginning of the year.