Authorities have often arrested critics of President Mugabe, who has ruled since 1980.
In March, two journalists were arrested over a report that the president, who travels abroad for regular medical treatment, was "in bad shape".
A pastor was also detained after prophesying that Mr Mugabe would die on October 17 this year.
Last week, police arrested a journalist who reported that Mr Mugabe's wife Grace had donated second-hand underwear to supporters.
Despite his advanced age and weakening health, Mr Mugabe's ruling Zanu-PF party has endorsed him as its candidate for the 2018 General Election.a