Morales, 59, South America's longest-serving leftist leader, won 45 per cent of votes, compared with Mesa's 38 per cent, according to the partial count, the Supreme Electoral Tribunal said late on Sunday.

Morales needed at least 40 per cent of votes with a 10-point lead over the runner-up to win outright. The final winner in the election will govern Bolivia, a landlocked country of 11 million people, from 2020 to 2025.

The results indicated that Morales would head to a second-round vote for the first time since sweeping to power in 2006, giving him his weakest mandate if he defeats Mesa in December.

Morales is running in defiance of term limits and despite a 2016 referendum in which Bolivians voted against allowing him to seek a fourth consecutive term. A local court ruling allowed him to run anyway. As he did in the 2014 election, Morales has promised to retire after the five-year term is over.

He has managed to hang onto power as most other leftist presidents in South America elected in the previous decade have since been succeeded by right-leaning leaders.

The former union leader for coca growers has overseen a long stretch of political and economic stability for Bolivia, the continent's poorest country.

But support for him has slipped amid slowing economic growth and concerns about government corruption and anti-democratic practices.

Australian Associated Press