POLICE fired tear-gas and detained nearly 1400 activists as demonstrators massed across Malaysia's largest city yesterday, demanding electoral reforms in the country's biggest political rally in years.

Organisers said 50,000 people had taken to the streets in the Kuala Lumpur protest.

Malaysian riot police officers fire tear gas during a rally calling for electoral reforms in Kuala Lumpur. Credit:AP

The opposition-backed rally was the culmination of weeks of intense pressure on Prime Minister Najib Razak's long-ruling coalition to make election laws fairer and more transparent before national polls expected by mid-2012.

Demonstrators marched in defiance of Mr Najib's administration, which had declared the rally illegal and repeatedly warned people to avoid it.