Tehran, Iran - Iran on Wednesday vowed fast-track trials for people arrested over a massive wave of protests, in response to US President Donald Trump's threats of "very strong action" if the Islamic republic goes ahead with hangings.

International outrage has built over a crackdown on nationwide demonstrations, which have led to hundreds of deaths, according to human rights organizations.

Iranian authorities have accused protesters of carrying out "acts of terror."

Judiciary chief Gholamhossein Mohseni Ejei said on a visit to a prison holding protest detainees that "if a person burned someone, beheaded someone and set them on fire then we must do our work quickly", in comments broadcast by state television.

Iranian news agencies also quoted him as saying the trials should be held in public and said he had spent five hours in a prison in Tehran to examine the cases.

Trump on Tuesday said in a CBS News interview that the US would act if Iran started hanging protesters.

"We will take very strong action if they do such a thing," he insisted, adding: "When they start killing thousands of people – and now you're telling me about hanging. We'll see how that's going to work out for them."

Iran called the warnings a "pretext for military intervention".

The US State Department and Amnesty International said 26-year-old Erfan Soltani had been sentenced to be executed on Wednesday.