Thailand's justice minister Chaikasem Nitisiri said the army had not consulted the cabinet on their decision to impose martial law, but appeared to play down the move, saying it was "only taking over security" and it would not affect the cabinet.
Protest leader Jatuporn Prompan said his group would accept the imposition of martial law, but "won't tolerate a coup or other non-constitutional means" to grab power.
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The Queen has arrived at Royal Manchester Children's Hospital to meet victims of the suicide bombing at the city's arena.
Bomb disposal units have been sent to a site in Manchester and are responding to an incident.
Across the nation a minute's silence was observed at 11am to remember the victims of the Manchester Arena terror attack.