Two Singaporean men have been arrested for staging a weekend protest outside the government complex that houses the offices of the president and prime minister, police said on Monday.

The men, aged 24 and 25, "turned up at the front gate of the Istana with placards" on Saturday afternoon, a Singapore Police Force spokesman told AFP.

"The duo refused to stop their activity despite officers requesting them to do so," the police spokesman said.

"The duo were subsequently arrested for organising a public assembly without a permit under... the Public Order Act," he added.

Police investigations "are ongoing", the spokesman said, without providing further details.

It was the second act of protest in the city-state since founding prime minister Lee Kuan Yew died on March 23.

Last Tuesday, a 16-year-old Singaporean student was charged with jailable offences for anti-Christianity slurs in a YouTube video criticising Lee, who ruled the country with an iron hand as prime minister from 1959 to 1990 before becoming an influential adviser in the cabinet.

Singapore has been hailed internationally for its economic achievements but the long-ruling People's Action Party, co-founded by Lee, has been criticised for clamping down on civil liberties, including the freedom of speech and assembly.

Outdoor protests outside Singapore's sole free-speech park are rare. Such events require police permits, which are rarely granted.

Photos circulated on social media showed the two protestors, clad in red hoodies and jeans, each carrying a large white placard while standing outside the main gate of the Istana.

One read "You can't silence the people", while the other read "Injustice".

The heavily guarded 106-acre (42.9 hectares) Istana complex, built by British colonial administrators, is currently the official workplace of the senior Lee's son, Prime Minister Lee Hsien Loong, and President Tony Tan.

It was also Lee Kuan Yew's workplace until his death at age 91 after a long illness.

His family held a two-day private wake at the Istana -- which means 'palace' in Malay -- before his remains were transferred to parliament to lie in state. He was cremated on March 29 after a grand state funeral.

Under Singapore's Public Order Act, participants in illegal protests can be fined up to Sg$5,000 ($3,700).

Organisers of such protests face a fine of up to Sg$10,000, a jail term of up to six months, or both.