Kuala Lumpur: Police are cracking down on social media sites that published criticism of Malaysia's long-ruling coalition that won last Sunday's national elections, and have banned a planned rally to protest the election.

Two men who posted Tweets have been arrested and bloggers are being targeted in police investigations as opposition leader Anwar Ibrahim vowed to lead a "fierce movement" to reform the country's legal system.

Malaysia's Prime Minister and Barisan Nasional chairman Najib Razak celebrates his victory on election day on May 6, 2013 in Kuala Lumpur, Malaysia. Credit:Nicky Loh

Social media sites played a key role in Mr Anwar's three party alliance gaining more than 50 percent of the popular vote on Sunday in the government's worst electoral performance since the country gained independence from Britain in 1957.

Malaysia's government-owned or controlled newspapers, television and radio refused to give anything but unfavourable coverage to opposition parties in the months before the election.