Malaysia's top opposition leader has warned the government that it may face a "hibiscus revolution" unless activists' demands are met for electoral reform and an end to "dirty politics".

Anwar Ibrahim's comments came a day after an estimated 20,000 people took to the streets to protest against alleged vote-rigging and other electoral abuses, defying a government ban and widespread use of teargas and water cannon filled with chemically laced water to deter the crowds.

More than 1,400 people were arrested and 12 injured — including Anwar — as clashes between police and protesters broke out in Malaysia's biggest protest since 2007.

Speaking to a small group of young graduates on Sunday, the People's Justice party leader warned prime minister Najib Razak's government: "We will have to pursue – in parliament and outside of parliament – free and fair elections, even by rallying unless they change the electoral vote."

When asked whether Malaysia was in the middle of its own revolution, Anwar said: "We have no confidence left [in the government], so it is important for Najib to consider seriously that we are close [to that]."

Najib, whose ruling party Barisan Nasional (National Front) has been in power since 1955, has dismissed Anwar as an opportunist who "will do everything, good or bad" to become prime minister himself. He has also claimed that Anwar might have been using Saturday's rally – organised by the Coalition for Clean and Fair Elections, a loose group of 62 non-governmental organisations – for his own political gain.

Najib's government took extreme measures to prevent people from attending Saturday's protest, including a city-wide lockdown that saw police suspend public transportation, seal off main roads, and threaten to arrest anyone wearing yellow, the colour of the coalition.

Amnesty International denounced the government's actions as "the worst campaign of repression we've seen in decades".

But Najib has since warned activists not to take to the streets again and claimed that his party, Umno, far outnumbered opposition groups.

"Don't doubt our strength," he said. "If we want to create chaos, we can. Umno has 3 million members. If we gather 1 million members, it is more than enough. We can conquer Kuala Lumpur."

Such comments – along with images and videos of police brutality against peaceful protesters – are undermining the BN's future, some say.

"They made a very big mistake on Saturday, by assuming that arresting more people would make the rest of us scared," said youth leader Khairul Anuar. "Actually, it has just made us more brave."

A similar rally in 2007 is widely credited with spurring on Malaysia's opposition movement, which won a landslide victory in the 2008 elections.

Many are hoping that Saturday's rally could potentially topple the BN entirely in the next election, slated for as early as this year. But some analysts say the government may hold off in order to smooth things over.

"From Najib's perspective, holding elections any time soon would be a mistake because of the damage that has been done," Bridget Welsh, Malaysia specialist at Singapore Management University, told Reuters.

"The fact that such a large crowd turned up despite a crackdown shows that voter anger is deep and this is going to push a lot of people who are in the middle towards the opposition."