Thailand's king has endorsed the army chief who seized power in a coup last week, amid widespread international criticism and increasing detention of those considered to be opposed to the takeover.

General Prayuth Chan-ocha told journalists on Monday morning that the much revered King Bhumibol Adulyadej, 86, had officially backed him as the leader of the military council now running the country.

Prayuth seized power on Thursday after six months of political in-fighting between the now-deposed government and its critics, who had taken to the streets and besieged government buildings in an effort to oust it. At least 28 people were killed and more than 700 injured in sometimes violent clashes after anti-government protests began in November.

Vowing to restore order across Thailand, Prayuth said he would have no choice but to use violence if anti-coup demonstrators, who over the past four days have taken to the streets in Bangkok, Chiang Mai and Pattaya, continued to protest.

"The most important thing right now is to keep peace and order in the country," said Prayuth, dressed in a white military uniform and flanked by 14 officers, in the televised press conference. "The less you allow me to speak, the more I'll be able to work."

The royal endorsement officially allows Prayuth to head the National Council for Peace and Order (NCPO), which will establish a legislative assembly and reform committee, draw up a temporary constitution, and appoint a prime minister. Prayuth promised elections would take place, but only when Thailand was "at peace".

One of the council's first actions will be to arrange payments to nearly 1 million farmers under a botched billion-dollar rice-pledging scheme initiated by the now-deposed Pheu Thai government.

Prayuth has enacted sweeping changes in the four days since he deposed the democratically elected government.

More than 200 journalists, academics, politicians and activists have been rounded up and many of them detained in undisclosed locations, ostensibly to give them "time to think", as Prayuth said last week. Although the army has said detentions will last no longer than a week, observers fear they are merely a means to stifle dissent against the takeover, because many of those summoned include members of the former cabinet, including the former prime minister Yingluck Shinawatra and her supporters, and those who appear to have been outwardly critical of Thailand's lese-majesty law, which protects the monarchy.

As of Monday, an unknown number of people still remained in detention, including the outspoken Thai journalist Pravit Rojanaphruk of the English-language newspaper the Nation, who has not been seen or heard from since arriving at army headquarters on Sunday, the newspaper reported.

Yingluck was released from detention late on Sunday but now has soldiers guarding her home and must inform the junta of her activities, a senior military official told Reuters.

Suthep Thaugsuban, who has led anti-government protests for the past six months, has also been released from detention but was handed over on Monday to the prosecutor's office on rebellion charges, along with 10 other leaders of the People's Democratic Reform Committee (PDRC), the Nation reported.

Another nine people – among them law and political science professors from Bangkok universities – were summoned late on Sunday night, the online news portal Prachathai reported, with some pro-government leaders and anti-coup protesters having been arrested in the northern city of Chiang Mai over the weekend.

The junta has said that the military now has the jurisdiction to intervene in all legal cases – including lese-majesty and national security cases – and has warned civilians and the media against posting anything on social media that could be deemed a threat to national security.

"For those of you who use social media to provoke, please stop because it's not good for anyone," the deputy army spokesman, Winthai Suvaree, said in a televised statement. Nineteen news editors were called in on Sunday for the junta to speak to them about covering news during an "abnormal situation", the Nation reported.

Various human rights groups and countries have expressed their concern over the coup – among them the US, the EU, Japan, France and Germany – and issued warnings against "non-essential travel" to the popular tourist destination. The US has suspended roughly one-third of its aid to the country.

The former education minister Chaturon Chaisang, who is currently in hiding, told Reuters by telephone that the junta was likely to tighten its grip as the days wore on.

"This is very serious indeed, it's very bad," Chaturon said. "It seems they'll detain a lot of people and we don't know for how long. It's going to be very oppressive."