A legal group on Tuesday labelled the state's move a "desperate bid to bolster its case."
Mtetwa – whose arrest for obstructing justice last month sparked international condemnation – now faces additional charges of abusing the police, according to Zimbabwe Lawyers for Human Rights.
Mtetwa was arrested a day after Zimbabwe's constitutional referendum, during a police raid of Prime Minister Morgan Tsvangirai's offices in Harare.
She is alleged to have said: "Stop whatever you are doing, it's unconstitutional, illegal and undemocratic. You confused cockroaches.
"You are Mugabe's dogs," she is also alleged to have told police in Shona, a local language.
It is a crime in Zimbabwe to insult the police or the office of the president.
State prosecutors presented the new charges in documents sent to Mtetwa's lawyers ahead of a May 27 trial.
Her arrest came a day after Zimbabweans voted overwhelmingly for a new Constitution that would enshrine civil rights and pave the way for a new government.
Police had initially ignored a court order for her release, prompting allegations that supporters of President Robert Mugabe were pursuing a politically motivated prosecution, a charge they deny.
Tsvangirai slammed the arrests of Mtetwa and four of his aides saying they were "meant to harass and intimidate the nation ahead of the election" expected later this year.
Mtetwa is now out on bail. – AFP