By Rozanna Latiff

KUALA LUMPUR (Reuters) - Malaysian police arrested the head of the pro-democracy group Bersih and several opposition leaders on Friday, widening a crackdown on government critics a day before a rally calling for Prime Minister Najib Razak to step down.

Thousands are expected to march on Saturday in a protest organized by Bersih, an electoral reform group, to demand Najib resign over his involvement in a multi-billion-dollar misappropriation scandal at state fund 1Malaysia Development Berhad (1MDB). Najib has denied any wrongdoing.

Bersih chairperson Maria Chin Abdullah and secretariat member Mandeep Singh were arrested shortly after the group's offices were raided.

City police said they were arrested for offences related to rioting. Laptops, bank and payroll statements were seized.

Human Rights Society chairwoman Ambiga Sreenevasan, Bersih's former chief, said in a text message the demonstrations would go on despite the arrests.

"I think we will have a larger turnout because of them," she said.

Last year, more than 200,000 people attended a similar rally organized by Bersih, which has held several mass protests calling for electoral and institutional reforms since 2007.

Five opposition leaders including Anthony Loke and S. Arutchelvan were arrested, civil activists and local media said.

Two student leaders, including Anis Syafiqah Md Yusof who in August organized a protest calling for arrests in the 1MDB scandal, were also detained.

"These arrests are the latest in a series of crude and heavy-handed attempts to intimidate Malaysian civil society activists and other human rights defenders," said Josef Benedict, Amnesty International's Deputy Director for Southeast Asia and the Pacific.

"They must be released immediately and unconditionally, and tomorrow's rally must be allowed to go ahead peacefully."

Bersih, along with other groups, is being investigated after reports that it received funds from Open Society Foundations, an organization linked to business tycoon George Soros.

Najib's administration has cracked down on the media and civil society in an attempt to silence criticism over his involvement in a money-laundering scandal at 1MDB.

Founded by Najib, who chaired its advisory board, 1MDB is the subject of investigations in at least six countries, including Switzerland, Singapore and the United States.

Lawsuits filed by the U.S. Justice Department in July say more than $700 million of misappropriated funds from 1MDB flowed into the accounts of "Malaysian Official 1", whom U.S. and Malaysian officials have identified as Najib.

(Reporting by Rozanna Latiff; additional reporting by A. Ananthalakshmi; Editing by Nick Macfie and Janet Lawrence)