A renegade Thai general who backs anti-government protesters has been shot, shortly after a deadline for troops to seal their Bangkok protest camp passed, BBC reported.
Khattiya Sawasdipol, better known as Seh Daeng (Commander Red), was shot in the head and seriously injured.
In clashes later, a protester was shot dead. The army has moved to seal off the protesters' large camp.
A state of emergency in place in the capital and surrounding areas is to be extended to 15 other provinces.
The BBC's Rachel Harvey in Bangkok says all talk of reconciliation and election timetables has been abandoned and the Thai capital is braced for further bloodshed.
The US has closed its embassy in Bangkok saying it is "very concerned" - and the UK also said it was closing its embassy on Friday because of the situation.
'Military strategist'
People were earlier urged to leave the area near the protesters' sprawling camp, and shops and businesses were advised to close before the 1800 (1100 GMT) deadline passed. Public transport was suspended in the area.
Some time after the deadline had passed, a volley of shots rang out and an explosion was heard.
Then reports came that Seh Daeng had been shot and rushed to hospital.
He is a suspended army officer who describes himself as the red-shirts' military strategist.
Seh Daeng is part of the protesters' more radical wing and had accused red-shirt leaders - many of whom have distanced themselves from him - of not being hard-line enough.
Circumstances surrounding the shooting, near the Silom business area, are not clear.
However, the New York Times reported that Seh Daeng was shot in the head during an interview with one of its reporters.
Sean Boonpracong, international spokesman for the red-shirt movement, told the BBC he believed an army sniper had shot the general.
A military spokesman, Col Sansern Kaewkumnerd, had earlier warned that sharpshooters armed with live ammunition would move into position in the area.
Government spokesman Panitan Wattanayagorn told the BBC that troops could only use their weapons in self-defence.
He said any "unusual" engagement would be investigated.
The second, and fatal, shooting happened as a group of more than 100 protesters advanced towards security forces, our correspondent says.
Street lights have been switched off in the protesters' camp, plunging parts of it into darkness, but they continue to defiantly blast out music, she adds.
Earlier in the day BBC reporters saw trucks unloading heavily-armed soldiers several blocks from the encampment, and later a group of about 200 soldiers moving towards it.
The decree extending the state of emergency to a further 15 provinces gives the army broad powers to deal with protesters.
Mr Panitan said the new measures were intended to prevent "masses of people trying to come to Bangkok".
Elections demanded
The protesters - who have been occupying parts of Bangkok for more than two months - want Prime Minister Abhisit Vejjajiva to dissolve parliament and call fresh elections.
Their camp stretches from the city's shopping district south to its business hub.
Mr Abhisit is under severe pressure to end the protests, which have paralysed Bangkok since 14 March.
He had offered polls on 14 November - but the two sides failed to agree a deal because of divisions over who should be held accountable for a deadly crackdown on protests last month.
The 10 April operation left 19 protesters, one journalist and five soldiers dead.