Egyptian activists fear the country's new army-backed regime will follow a crackdown on Islamist allies of the former president, Mohamed Morsi, with a wider campaign against those opposed to both Morsi and the military.

It follows the arrest of one of the leading activists from the 2011 revolution on charges of attempting to bring down the government, and the news – published in Egypt's flagship state newspaper, and then later denied – that 35 other prominent secular activists are also under investigation.

Haitham Mohamedein, a high-profile lawyer and key figure in Egypt's labour and revolutionary movements, was released without charge after being arrested and taken to a police court on Friday. The state prosecution office also denied to the Guardian on Sunday that any of the 35 other activists were at risk of legal proceedings.

But activists feared both instances indicated an attempt by the state to intimidate anyone likely to question the idea that the army's heavy-handed governance is the only alternative to Morsi's Muslim Brotherhood. Egypt's new army-backed government currently has widespread public support for its crackdown on Morsi's Muslim Brotherhood – a status quo those in power are keen to maintain.

"There is a crackdown on all those who oppose military rule," said Wael Abbas, a well-known journalist and rights activist initially listed on Saturday as one of the 35 activists under investigation. While officials categorically denied on Sunday that he and others were currently at risk, Abbas argued it was likely they had leaked information about a potential investigation to gauge public support for a widening of a crackdown on dissidents that has previously centred mainly on Islamists.

"It's a technique they've been using since the revolution," said Abbas. "We call it a test balloon."

Mohamedein's arrest and court hearing also suggested that the government was keen to frighten those involved in Egypt's influential workers' movement, Mohamedein's allies argued. Thousands of Egyptians – some of them represented by Mohamedein – have been on strike in recent weeks in the industrial cities of Suez and Mahalla in protest at poor conditions, and the government is concerned that the strikes might spread. Strikes played a key role in the downfall of Hosni Mubarak in 2011, and were a destabilising force during the final months of Morsi's tenure.

"It's really scary for the state and they do not want this to happen," said Tarek Shalaby, a colleague of Mohamedein's in Egypt's Revolutionary Socialist party. In an apparent attempt to smear all opposition to the state with the same brush, Mohamedein was asked in court if he was a Brotherhood member, despite having opposed Morsi's rule.

Comparing Egypt's current climate to the McCarthyist witch-hunts of 1950s America, Shalaby added: "They're scared of people thinking that this is not a binary situation. They want to make sure that people who are opposed to both the Muslim Brotherhood and the military are not a threat."

Mohamedein's was not the only arrest to concern campaigners this weekend. The Sinai-based journalist Ahmed Abu Draa – critical of the army's recent behaviour in the restive peninsula – was also detained, and both detentions are seen as just the latest symptoms of the new government's increasing authoritarianism.

They follow the public rehabilitation of secret police units nominally disbanded after the fall of Hosni Mubarak; heavy-handed policing that has seen hundreds killed in state-led massacres; the use of unregulated military trials to fast-track the sentencing of Muslim Brotherhood members; and moves to alter Egypt's constitution to allow senior Mubarak-era officials to return to politics.

Morsi's own year in office was no better. His presidency was also characterised by police brutality, persecution of political activists and journalists, and a failure to curtail either army influence or condemn police abuse.

• Additional reporting by Marwa Awad