The country's main political parties, including Mugabe's long ruling ZANU-PF, are in favour of the proposed law changes, making the simple majority needed for a "yes" vote a near certainly.

Voting kicked off at a slow pace with some short queues outside polling stations.

In Chitungwiza, a sprawling town on the fringes of the capital, as few as three people lined up to cast their ballots.

In Harare's flashpoint township of Mbare, where violence broke out on the eve of polling, 20 voters queued in line.

But Takawira Marimo, a 47-year-old handyman on his way to the second city of Bulawayo, stopped by at polling station in Mbare.

"I have done the right thing, I have exercised my right. I wanted to vote before I start my work today," he said.