Police arrested 21 Malaysians after they were caught staging an outdoor protest at the Merlion Park at 4.50pm on Saturday.
This despite earlier police advisories that such gatherings are against Singapore's laws.
"The police would like to reiterate (that) while foreigners are allowed to work or live here, they have to abide by our laws," said a police spokesman in a statement on Saturday. "They should not import their domestic issues from their countries into Singapore and conduct activities which can disturb public order, as there can be groups with opposing views. Those who break the law will be seriously dealt with,"
The arrest comes after warnings were issued to nine Malaysians for "actively participating in the illegal gathering" at the Merlion Park on Wednesday, where a crowd of about 100 people had gathered to protest against the recent Malaysian election results.
The organiser of another similar protest, which had been set for Saturday, had also cancelled it after being advised to do so by the police.
In a previous statement released on Friday evening, the police said the work and visit passes of nine Malaysians who were involved in the protest were being reviewed by the authorities. "Their employers will be informed of this. Further investigations are ongoing," added the police then.