Singaporeans must welcome visitors or risk lowering their standing in the eyes of the world, Prime Minister Lee Hsien Loong said in a strongly-worded Facebook post on Saturday.

"We must show that we are generous of spirit and welcome visitors into our midst, even as we manage the foreign population here. Otherwise we will lower our standing in the eyes of the world, and have every reason to be ashamed of ourselves," he said.

His comments come amid reports that plans to hold a carnival on June 8 at Ngee Ann City to mark Philippines' 116th Independence Day drew a storm of vitriol.

Negative comments from Singaporeans flooded in after event organisers posted an announcement on Facebook last weekend, with a page "Say 'No' to an overpopulated Singapore" urging locals to protest.

The organisers purportedly received expletive-laden phone calls, and their Facebook post also drew a barrage of anti-foreigner comments, leading them to take it down.

Saying he was apalled to read the news reports, PM Lee called the harassers' actions a "disgrace to Singapore", and said he was ashamed of the behaviour.

"Fortunately, this appears to be the work of a few trolls," Mr Lee added, pointing out that "many sensible Singaporeans condemn this thuggish behaviour" and support Acting Manpower Minister Tan Chuan-Jin's stand on the issue.

On Friday, Mr Tan had called on Singaporeans to say no to bigotry. "These actions by those who peddle hate are not acceptable, repulsive even," he wrote.

In a similar vein, Mr Lee urged Singaporeans to "treat people in Singapore the way we ourselves expect to be treated overseas".

"Many Singaporeans live overseas, and are warmly welcomed in their adopted homes. I just attended our Singapore Day in London. How would we have felt if British netizens had spammed our website, and abused Singaporeans living in Britain?"