Ousted members of Thailand's former government have turned themselves in to the country's new military junta, as soldiers forcefully dispersed hundreds of anti-coup activists who defied a ban on large-scale gatherings.

At least two activists were detained by troops during the protest in Bangkok city centre, which marked one of the first open challenges to the military since yesterday's coup. It descended into scuffles but ended without injury.

The junta remained firmly in charge, summoning more than 100 senior political figures - the entire ousted government, their associates and a handful of their opponents. It also banned those on its wanted list from leaving the country.

Among the officials who showed up at an army compound in Bangkok were former prime minister Yingluck Shinawatra, sacked earlier this month for nepotism by the Constitutional Court, and her temporary replacement Niwattumrong Boonsongpaisan, according to Ms Yingluck's aide Wim Rungwattanachinda.

After about 30 minutes, Ms Yingluck left the facility and was taken to another army location by soldiers, said the aide, who later added that it appeared she would not be immediately released.

It was unclear what the military's intentions were beyond the summons, which it said had been issued "to keep peace and order and solve the country's problems".

By nightfall, dozens of the VIPs who turned themselves in were still being held, although at least eight ex-cabinet ministers had been released.

One, education minister Chaturon Chaisang, an outspoken critic of the military's intervention in politics, remained in hiding. Mr Chaturon said in a Facebook post that the coup would only worsen the country's political atmosphere. He vowed not to turn himself in but said he would not resist arrest.

Most of the country was calm, and there was little military presence on Bangkok's streets. Although life had largely returned to normal during the day, an overnight curfew from 10pm to 5am was still in effect.

There were no reports of any unrest, including in the former government's political strongholds in the north. In the north east city of Chiang Mai, about 100 anti-coup demonstrators also took to the streets, but no violence was reported and the protesters dispersed on their own.

The army staged the coup yesterday just after a military-hosted meeting of political rivals to resolve the country's political deadlock.

After two hours and no resolution, armed soldiers detained the participants, including four cabinet ministers, and army chief General Prayuth Chan-ocha appeared on national television to announce the takeover. Hours later, the junta had suspended the constitution and banned gatherings of more than five people.

Gen Prayuth justified the coup as a necessary move to restore stability amid increasing spasms of violence that together with controversial court rulings had rendered the government powerless.

But there are fears the power grab will lead to more violence and deepen the nation's crisis.

The dramatic events were the latest response to a societal schism laid bare after the 2006 coup deposed former prime minister Thaksin Shinawatra, the older brother of Ms Yingluck and a billionaire tycoon whose populist movement has won every national election since 2001. He lives in self-imposed exile to avoid corruption charges, but he still wields enormous influence over Thailand's political affairs and remains at the heart of the crisis.

It is a divide that has led to upheaval multiple times in recent years. The latest crisis has left 28 people dead and more than 800 wounded since November.

US secretary of state John Kerry condemned the takeover and warned it would "have negative implications for the US-Thai relationship", but did not announce immediate punitive steps. The State Department said it was reviewing millions of pounds in aid.

British Foreign Secretary William Hague said his country looked to Thai authorites "to set out a quick clear timetable for elections to help re-establish the democratic framework of governance".

"There should never be recourse to violence. Only by openly discussing the full range of issues can Thailand move forward and reach a more stable position," he said in a statement issued by the Foreign Office.