Hundreds of riot police have moved up to confront red-shirt anti-government protesters in Bangkok's city centre, BBC reported.
But the police stepped back from immediate confrontation after a night in which five explosions killed at least one person and wounded 75.
Britain, the US and Australia have issued new warnings against travelling to Thailand after another night of violence and tension.
Six weeks of protests are severely challenging the Thai government.
United Nations Secretary General Ban Ki-moon said through a spokesman that it was "a moment requiring restraint on all sides".
The British government is advising against all but essential travel to Bangkok.
It said "this reflects its concern for the safety of British nationals... given that violence could break out without warning during the increasingly volatile political crisis".
While the British advice focused on Bangkok only and did not apply to transit travellers, the US and Australian warnings advised against travel to Thailand.
Thai emergency services have refused to confirm the government's claim that three people died in Thursday night's violence.
They say one woman died; among those injured were an Australian, and American and an Indonesian.
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