BANGKOK (AFP) - Thailand's new military junta summoned the kingdom's ousted leaders on Friday as an international outcry grew over a military coup that has brought a harsh crackdown on civil liberties.

Claiming they were acting to halt months of deadly political turmoil, coup-makers led by the tough-talking army chief declared a nationwide night-time curfew and ordered masses of rival demonstrators off the streets.

The military regime headed by General Prayut Chan-O-Cha suspended most of the constitution, drawing rebukes from Washington, Europe and UN Ban Ki-moon, who all called for civilian control to be restored.

Secretary of State John Kerry said there was "no justification" for a coup that would have "negative implications" for US relations, and demanded early elections. The Pentagon said it was reviewing military cooperation with America's oldest ally in Asia.

Southeast Asian neighbours urged caution with Malaysia warning its nationals to defer non-essential travel to Thailand.

Japan, Thailand's biggest foreign investor, stopped short of a travel warning but made its concern clear, as Toyota and Honda curtailed night-time shifts at their Thai plants because of the curfew.

"It is regrettable that this kind of event has happened, and our country wants to call strongly for a prompt restoration of a democratic political system," Japan's Chief Cabinet Secretary Yoshihide Suga said.

Undaunted, the military ordered members of the ousted cabinet, along with leaders of the Puea Thai party that had been in power, to report to the new regime in central Bangkok at 10:00 am (11am Singapore time) on Friday.

Former prime minister Yingluck Shinawatra - who was removed from office earlier this month in a controversial court ruling - was also ordered to show up, along with more than 100 prominent figures from both sides of the political divide.

It was unclear what awaited them.

Thailand has been locked in a nearly decade-long political crisis since a 2006 military coup that deposed Yingluck's elder brother Thaksin Shinawatra, a billionaire tycoon-turned-populist politician who clashed with the royalist establishment before his overthrow.

The military held on to power for more than a year after the 2006 coup and since then, a power bloc centred on Thaksin's family has battled for primacy with a Bangkok-based royalist camp closely tied to the powerful military.

His supporters, known as the "Red Shirts", had warned that an overthrow of the government could trigger civil war and all eyes were on how the movement would respond to the coup.

Prayut said the army takeover was staged "in order for the country to return to normal quickly".

"All Thais must remain calm and government officials must work as normal," he said in a brief televised address announcing the takeover on Thursday afternoon, flanked by top military and police officials.

All television and radio stations including foreign broadcasters were ordered to air only a steady stream of army announcements, and the junta warned it would block social media platforms that carry anti-coup content.

Bangkok was calm on Friday although its usually bustling streets were quieter than usual with schools ordered to temporarily close across the country.

In contrast to the previous coup eight years ago, there was no sign of tanks or significant troop numbers deployed around the capital.

A day earlier as the coup unfolded, there were dramatic scenes at a military-hosted meeting between the kingdom's political rivals as army trucks blocked exits.

Inside, Prayut abruptly announced he was taking power after the two sides failed to reach a compromise, according to an official at the talks who did not want to be named.

"Because you cannot agree and the situation is likely to escalate into violence I declare that I seize power, so soldiers detain everyone inside this room," the source quoted Prayut as saying.

"Don't fight with soldiers," the commander-in-chief warned, before armed troops detained those present one by one.

Election officials and senators were later released while others, including the leaders of the two main political parties as well as the rival protest leaders, were taken away to an undisclosed location.

Their whereabouts remained unknown early Friday.

Thailand's democratic development has now been interrupted by 19 actual or attempted coups since 1932, interventions that traditionally require the monarchy's approval.

It was unclear whether the palace had blessed Prayut's coup.

Some observers see the crisis as a struggle to decide who will run the country when the more than six-decade reign of ailing, 86-year-old King Bhumibol Adulyadej eventually ends.

Experts at the Siam Intelligence Unit, a Bangkok-based think-tank, expect an interim premier to be named and the junta to rule for up to two years and draft a "draconian" new constitution.

Some fear more turmoil.

"The coup is not a solution at all to end the crisis. This will become the crisis," said Pavin Chachavalpongpun of the Centre for Southeast Asian Studies at Japan's Kyoto University.

"It shows the military has never learned the lesson from 2006," he said, referring to the cycle of instability stemming from Thaksin's overthrow.