Zimbabwe's parliament has outlawed criticism of the government ahead of presidential and parliamentary elections in August, with violations of a new law punishable by up to 20 years in jail.

The Criminal Law Code Amendment Bill, widely known as the "Patriotic Bill", contains a clause that criminalises "wilfully damaging the sovereignty and national interest of Zimbabwe".

Opposition activists said the law, passed late on Wednesday, was designed to punish citizens, civil society organisations and political adversaries of the ruling ZANU-PF party.

It has raised fears that the government could launch a crackdown on dissent ahead of the general election on August 23, where President Emmerson Mnangagwa will be seeking a second term.

The 80-year-old's main rival is lawyer and pastor Nelson Chamisa, 45, who leads the newly formed Citizens Coalition for Change (CCC).

CCC spokesperson and lawyer, Fadzayi Mahere, described the law as "dangerous" and said it aimed at closing the democratic space ahead of elections.

"ZANU-PF has reduced our great nation into an outpost of tyranny," Mahere told Reuters.

"None of it will work because Zimbabweans go to the polls with one mission - to win Zimbabwe for change. No amount of panicky despotism by ZANU will stand in the way of change whose time has come."

A ZANU-PF spokesperson did not immediately respond to a request for comment.