The military also suspended the Constitution, declared a night-time curfew across the country and ordered all television and radio stations to stop their usual programmes and replace with the the army's statements.
Making the announcement of military takeover, army chief Gen Prayut Chan-O-Cha in a nationally televised address said the step was necessary to prevent the conflict from escalating.
"In order for the country to return to normal quickly the National Peace Keeping Committee comprised of the army, the Thai armed forces, the Royal Air Force and the police need to seize power as of May 22 at 4.30 pm (local time)," he said.
The military - which has attempted 18 coups, including 11 successful ones, since the end of absolute monarchy in 1932 - acted after political violence claimed 28 lives and left hundreds wounded.
He asked all Thais to remain calm and said all government officials must work as normal.
Declaring the night-time curfew, the military said: "Under martial law, the National Peace Keeping Committee prohibits anyone across the kingdom from leaving their home from 10 pm to 5 am."
The move was to ensure the release of "accurate news to the people," an army spokesman said in a televised announcement.
The military leaders said the national Constitution had been suspended and ordered rival protesters to disperse and go home.
"In order to run the country smoothly, (coup leaders have) suspended the constitution of 2007, except for the chapter on the monarchy," said a statement read out on national television.
"In order to keep peace and order all protesters from every group must leave for their homes immediately," an army spokesman said in a televised address, adding that buses had been arranged to take them away.
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