Dozens of social media users in Pakistan have been detained or interrogated by the Federal Investigation Agency (FIA) for posting "anti-military" content, opposition parties and activists say.

On Friday, the FIA reportedly compiled a list of 33 social media users for carrying out propaganda against the army, a report in The Nation said.

Interior Minister Chaudhry Nisar Ali Khan had earlier asked authorities to take stern action against social media users posting negative content to malign the armed forces, according to Radio Pakistan.

At least 23 Pakistan Tehreek-e-Insaaf (PTI) supporters were detained and threatened with action under the Prevention of Electronic Crime Act, Fawad Hussain Chauhdry, a spokesman for the party, told AFP news agency on Monday.

It is the first time the new law has been used in a broad crackdown against political opposition.

Two peope were charged under the law, including one for sharing a satirical picture of Prime Minister Nawaz Sharif, and are on bail pending further investigation.

"One of our supporters was abducted from Quetta and brought to Islamabad in an armoured car. People are being harassed and it is spreading fear," the spokesperson said.

He added the party would sue the government for intimidation and harassment in the Islamabad High Court.

In addition to political activists, a Pakistani journalist said he had received an intimidating phone call from an alleged FIA official who asked him to appear in person to explain his social media activity.

"Earlier, I was told through informal channels that I should reconsider posting comments and opinions on social media related to my work on the military since they were crossing a line," Taha Siddiqui, the France24 correspondent in Pakistan, told Al Jazeera.

"Now they are using counterterrorism department to silence me. I'm in no way trying to malign the army as it's being portrayed."

READ MORE: Disappeared - Silencing Pakistan's activists

The interior ministry declined to comment on the cases.

After visiting the FIA headquarters on Sunday, PTI Vice President Shah Mahmood Qureshi said the party "stood by our [detained] activists and their families. Freedom of expression and democracy go hand-in-hand."

A spokesperson for the Pakistan Peoples Party called the action "unacceptable" before adding that it was "disturbing that in a short span of time nearly one thousand cases have been registered against social media platforms".

Nighat Daad, executive director of Digital Rights Foundation, said the detentions show a strong political movement that was currently taking place.

"Social media is not entertainment any more, there's a strong political movement happening here before the elections," Daad said.

"But this is a democracy, not dictatorship. There should be a process that has to be followed."

Disappeared activists

Pakistan's parliament passed the cybercrime law last August, despite opposition from rights activists which said its wording was overly broad and would curb free speech.

Of particular concern was a clause that empowered the government to ban speech considered "against the glory of Islam or the integrity, security or defence of Pakistan".

In January, five activists known for their outspoken views against religious extremism and the powerful military disappeared.

Four of them were returned to their families weeks later, but not before they were tarnished by a virulent campaign to paint them as enemies of Islam deserving execution.

One of them later told AFP they had been held and tortured by the shadowy Inter Services Intelligence agency.