Sten Zvorwadza was uninjured after he was punched by a group of youths in Harare, the BBC reported.
The youths also attacked a BBC reporter and his crew at the scene.
Zvorwadza is a candidate for parliament with the Movement for Democratic Change party, which has been part of a coalition government with the ruling Zanu-PF party of President Robert Mugabe for the last four years.
Under the proposed constitution, the person who is elected president in July would be limited to two terms.
Mugabe has been in office since 1980.