More than 1,000 Singaporeans attended the city-state's biggest protest rally in recent memory Saturday, amid growing public indignation over predictions of a surging foreign population.

The peaceful rally, held at an officially designated protest zone, was staged by a civic group after the government said foreigners could account for nearly half of the densely packed island's population in less than 20 years.

Organisers estimated the crowd at 3,000, but AFP reporters on the scene said between 1,000 and 1,500 people had taken part despite afternoon downpours, making it the biggest protest in Singapore in recent years.

Rally leaders, who used Facebook and other online platforms to draw support, openly attacked the People's Action Party (PAP), which has been in power for more than 50 years, over its immigration and other policies.

"The large crowd here shows the PAP government that they are not afraid any more, they don't want to hide behind a moniker on Facebook to show their displeasure," said chief organiser Gilbert Goh, a former opposition candidate for parliament.

"They are showing their deep displeasure with the white paper," he told AFP, referring to a controversial population projection issued last month.

A spokesman for the Singapore Police told AFP that it was not monitoring the size of the crowd, which was largely clad in black, armed with clappers and clutched a sea of umbrellas.

There were no signs of riot police in or around the rally venue, a grassy park where protesters are allowed to address the public in a spot known as Speakers' Corner.

A government policy paper last month said the population could range between 6.5 and 6.9 million by 2030, with foreigners making up 45 percent because Singaporeans are not producing enough babies to sustain economic growth.

Citizens currently make up 62 percent of the current population of 5.3 million, of whom more than a third are foreign-born.

A banner at the rally read: "Save Singapore - Say NO to 6.9 million."

"Stop selling memberships. We are not a country club," read a handmade poster, referring to naturalisation.

Protests are rare in Singapore, a wealthy island republic known for strict security and social controls, but Facebook, Twitter and other social media have set the tone for political debate in recent years.

Rallies of more than a few dozen people are unusual.

In October 2008, about 600 angry investors gathered in the same spot, urging the Singapore central bank to help them recover money they lost from investments linked to collapsed US bank Lehman Brothers and other institutions.

Saturday's rally came less than two years after the general election of May 2011, when the ruling party suffered its worst ever performance, with immigration already a sensitive issue.

Foreigners have been blamed for stealing Singaporeans' jobs as well as straining housing, transport and medical services.

Goh, the rally organiser, ran unsuccessfully for parliament in 2011 under the opposition National Solidarity Party.

On the eve of the rally, Goh issued a public apology to foreigners living in Singapore for an earlier posting in which he listed racial profiles and social habits of various Asian nationalities as well as westerners.

"It's rude and insensitive to their feelings," Goh wrote, adding that he did not want to "stir up unpleasant xenophobic sentiments within the country".