Twenty-one Malaysians arrested in Singapore for staging an illegal protest following disputed elections in their neighbouring homeland have been released on bail, police said on Tuesday.

"They have been released on bail. Police investigations are ongoing," the Singapore Police Force said in a statement sent to AFP.

The 21 arrested comprised of eight males and 13 females, aged from 21 to 40 years old, the statement said. No further information was given.

The protesters were detained on Saturday after staging a rally at a park along Singapore's Marina Bay promenade, despite police warnings not to repeat a protest held on the same spot three days earlier by some 100 Malaysians.

No arrests were made in the first rally, but police warned foreigners to adhere to Singapore's laws against illegal protests.

That initial rally coincided with a mammoth protest in Kuala Lumpur by the Malaysian opposition, which
claims it was robbed of victory in the May 5 parliamentary elections through fraud.

Under Singapore's Public Order Act, organisers of illegal protests can be jailed for up to six months or fined Sg$10,000 ($8,100). They can also be both jailed and fined.

Participants can be fined up to Sg$5,000. Foreigners jailed in Singapore are normally deported after serving their sentences.

The Malaysian opposition, led by former deputy prime minister Anwar Ibrahim, has called for a wave of
protests after the ruling coalition was returned to power despite receiving less than 50 percent of votes cast.

Malaysians overseas, including in Australia and Taiwan, have also held rallies to protest against the election results.

Singapore was ejected from the Malaysian federation in 1965 after a brief union between the two former
British colonies, but they are bound by strong economic, political, cultural and family links.