(CNN) -- The largest political protest in nearly a decade erupted in Malaysia's capital city, Kuala Lumpur, Saturday with riot police aiming water hoses and tear gas at thousands of protesters gathered to demand electoral reform.

Tens of thousands of protesters took to the streets of Kuala Lumpur to demand electoral reform.

Opposition parties and civic groups demonstrated against alleged fraudulent activity in the electoral process and demanded an overhaul of Malaysia's electoral commission ahead of general elections widely expected for early next year.

Malaysian Prime Minister Abdullah Ahmad Badawi had vowed to suppress the demonstration, and on Saturday police had erected roadblocks and ramped up security in an attempt to close down the city's center.

Nevertheless, in defiance of a government ban, between 30,000 and 40,000 demonstrators massed outside the royal palace in Kuala Lumpar, according to media reports. Opposition group leader and former deputy prime minister Anwar Ibrahim put the number much higher, claiming more than 100,000 people had gathered in the streets.

One witness said police fired tear gas and jets of "chemically-laced water" at hundreds of demonstrators who sought refuge in the city's Jamek mosque and in commercial buildings.

"Squads of police are chasing hundreds of protesters along alleys and on the city streets," the witness said, speaking on condition of anonymity. He said blockades had been set up around the city to hem in demonstrators.

Photos of the crackdown showed protesters dressed in yellow t-shirts and head scarves shielding their heads as water from cannons blasted down on them.

New York-based Human Rights Watch slammed the rally ban and urged the government to support free speech ahead of elections expected to be called early next year.

"If Malaysia wants to count itself a democracy, it can begin by upholding constitutional guarantees of free speech and assembly. The way the system works now, only the ruling coalition can get its messages out," it said.

Human Rights Watch said Malaysian elections have been sullied by vote-buying, the use of public resources by the ruling parties and accusations of bias against the Election Commission.

Malaysia has had only one party in power since 1957.

Speaking to CNN after briefly addressing the opposition-backed rally, Anwar said "we are demanding that the (election) process be cleansed. There are no such thing as fair elections in Malaysia at the moment."

He said a memorandum detailing allegations of corruption by the commission had been handed to Sultan Mizan Zainal Abidin, Malaysia's constitutional monarch.

Malaysian law stipulates the sultan must give his royal assent to the commission after it has been appointed by the government.

Opposition party leaders, including Anwar, called the mass meeting to protest alleged fraudulent activity in the electoral process.

"This was an attempt to threaten the people. But I am very proud that Malaysians were not intimidated and turned out in such great numbers and that they behaved peacefully," Anwar said.

Anwar was heir apparent to former premier Mahathir Mohamad until 1998, when he was sacked and charged for corruption and sodomy.