Thailand's military junta has dissolved the country's last remaining democratic institution – the senate – and ordered dozens of activists and journalists to turn themselves in to the army as protests grow against Thursday's coup.

The governing National Council for Peace and Order (NCPO) announced in a televised statement late on Saturday that it had dissolved the senate and would be assuming control of all lawmaking powers, a few hours after it summoned 35 prominent academics and activists to report to army headquarters. That was in addition to some 155 leading politicians and leaders it had already called in for questioning.

The council has also sacked the police chief and head of the department of special investigations, Thailand's FBI, and on Sunday afternoon it called in the editors of 18 major Thai newspapers. They included Khaosod, the Bangkok Post, ASTV, Matichon and Thairath, according to the online news portal Prachatai. It was not clear if the editors would be allowed to leave or detained in unknown locations like those arrested earlier.

In a separate summons, the outspoken columnist Pravit Rojanaphruk, who writes for English-language paper the Nation, was asked to meet independently with the junta and said on Sunday morning he was en route to army headquarters. "On my way to see the new dictator of Thailand. Hopefully the last," he tweeted. He did not appear to have been released by Sunday afternoon.

Yingluck Shinawatra. Photograph: Narong Sangnak/EPA

More than 100 people, among them the former prime minister Yingluck Shinawatra, who was apprehended on Saturday, remain in military detention in secret locations in what has been seen as a move to suppress dissent and potential opposition to the military takeover.

"What is scary now is that the army appears to be the most united institution in Thai society," said Dr Paul Chambers of the institute of south-east Asian affairs, which is affiliated to Chiang Mai University.

Thailand's army seized power in what it said was an effort to restore peace and order after six months of political infighting left 28 people dead and more than 700 injured. But the coup leaders are also facing trouble in southern Thailand, where at least 10 bombs exploded on Saturday, reportedly killing three people and wounding 63. An Islamic insurgency has been under way in the south for a decade.

Troops were deployed across central Bangkok on Sunday, concentrated around the shopping malls of Rajprasong, to dispel a growing crowd of protesters defying a public ban on gatherings of five or more people. The crowd shouted at the soldiers, held up banners reading "anti coup" and called for elections and a return to democratic rule.

Soldiers stand guard in Bangkok after dispersing anti-coup protesters on Sunday. Photograph: Christophe Archambault/AFP/Getty Images

About 1,000 people gathered for a similar protest on Saturday in Bangkok, where some were reportedly arrested, while others protested against the coup in the northern city of Chiang Mai, a political stronghold for the now-deposed Pheu Thai party. Demonstrations also took place at the beach resort of Pattaya and at Khon Kaen in the north-east.

The army chief and head of the junta, General Prayuth Chan-ocha, now serving as acting prime minister, warned people against joining the protests: "Democratic principles cannot be executed normally," he said.

On Thursday, Prayuth dissolved the lower house of parliament and suspended the constitution, one that he had previously helped draw up in 2006 when he was part of the last military coup that deposed Thaksin Shinawatra as prime minister. Emptying out both the house and senate means the junta will bypass the need for parliamentary approval of new laws.

Many of those told to report to the junta appear to been outwardly critical of Thailand's lese-majeste law, known as Article 112, which calls for up to 15 years' imprisonment for defamatory, insulting or threatening comments about the royal family. Critics claim it stifles public debate and helps to maintain the status quo.

Recent attempts to reform the law have been met with fervent resistance by royalists, among them Prayuth, who according to the Associated Press told critics: "If you guys play hardball I'll have no choice but to do so too."

After initially blocking all Thai and foreign television channels from broadcasting, the junta has allowed some to go back on the air. But it appears that many international channels – among them CNN and BBC – are still blocked. About 14 Thai TV channels and radio stations were still banned, the Bangkok Post reported.

Chambers said the speed at which the junta was instituting change was "very worrisome" and that the immediate future for Thailand looked bleak. "I see more jailings. I see the enshrinement of more army power. I see the beginning of civil war after Thaksin announces a government in exile," he said.

A number of nations and human rights organisations have expressed their grave concern over the military's recent actions. The US, a major trading partner, has suspended $3.5m (£2m) in military aid.

"We are increasingly concerned about actions the military has taken, just a few days after it staged a coup … We again call on the military to release those detained for political reasons, end restrictions on the media and move to restore civilian rule and democracy through elections," a state department spokeswoman said on Saturday.

Thai police commandos stand guard outside the Army Club in Bangkok on 23 May. Photograph: Sakchai Lalit/AP

The Asian Human Rights Commission said the military's use of the media to issue the summons could be seen as a form of terror, and said that those in detention in were in danger of being subjected to grave human rights abuses.

"While the junta has made reassurances that those who report themselves will not be mistreated, within the context of martial law … this reassurance carries no weight," it said in a statement.

"Citizens are subject to up to seven days of detention without the authorities having to provide evidence of wrongdoing or bring formal charges … mean[ing] that the possibility for rights violations, including torture, forced disappearance or extrajudicial execution is greatly increased."