Kuala Lumpur: Critics of Malaysia's electoral system called on Monday for a nationwide protest this month to press for thorough reform before upcoming polls that they warned could be "the dirtiest yet".

Prime Minister Najib Razak is expected to seek a fresh mandate within months but reform activists Bersih 2.0 said the electoral system remains riven with fraud despite reform pledges and called for a "peaceful sit-in" on April 28.

The group brought tens of thousands into the streets of the capital Kuala Lumpur last July in a rally, broken up by police, that placed the issue of alleged pro-government electoral bias squarely on the national agenda.

A statement by Bersih 2.0 said the demonstration will "express our deep disappointment with a government that continues to manipulate the electoral system to their advantage."

"In fact, it promises to be the dirtiest (election) yet," it said.

The group said the rally would be held at Independence Square, the historic centre of Kuala Lumpur, but urged protests by supporters nationwide.

Bersih 2.0 called for the country's Election Commission -- accused by critics of favouring the ruling coalition – to resign, saying it had lost public confidence.

They also demanded a thorough vetting of the nationwide voter roll, for major reforms to be completed before elections are called, and for international observers to monitor the polls.

The calls came a day after parliament endorsed the findings of a panel set up by Najib to explore reform options.

The panel urged independence for the Election Commission, a clean-up of the voter roll and other steps, but critics said the report was short on specifics and deadlines and ignored many key reform demands.

Najib set up the panel in October under pressure for reform and amid criticism both at home and abroad for the harsh response to last July's rally, in which police used tear gas and water cannon and arrested 1,600 people.