Belarus' President Alexander Lukashenko has said a deadly blast in the metro system in the capital Minsk was an attempt to destabilise the country.
Eleven people were killed and scores injured in an explosion during the evening rush hour on Monday.
"I do not rule out that this [blast] was a gift from abroad," said Mr Lukashenko, linking it to an explosion in 2008 in which 50 people were hurt.
A police spokesman said Minsk had been placed on high alert.
'Links in a chain'
The explosion tore through the Oktyabrskaya metro station, about 100 metres from President Lukashenko's main office and residence, at 1755 local time (1555 GMT). The station, one of the busiest in Minsk, links the city's two metro lines.
Witnesses said there was a flash and a bang as passengers were getting off a train.
Many people suffered serious injuries - Interfax reported the explosive device was packed with metal fragments - and some witnesses spoke of bodies piled on the platform.
In televised remarks, Mr Lukashenko said the explosion was aimed at undermining "peace and stability".
And he hinted at foreign involvement, making a connection to a blast at an independence day concert in 2008, when about 50 people were injured. That crime was never solved.
"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.
He called for a moment of silence to honour those killed.
"Prosecutors qualify this as a terrorist act," a source in Mr Lukashenko's administration told the Reuters news agency.
One witness said at least part of the station's ceiling had collapsed after the explosion.
Shortly 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."
Mr Lukashenko, who has ruled Belarus since 1994, claimed victory in a presidential election last December, but international observers condemned the vote.
When the opposition called a rally in protest, about 600 people - activists and presidential campaigners - were rounded up and arrested. Some are still in jail.
The European Union and the United States imposed a travel ban on Mr Lukashenko and his inner circle because of the crackdown on 19 December.
Mr Lukashenko has previously said the opposition rally was an attempted coup financed by the West.
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.