The police announced that they had smashed an Indonesia-backed conspiracy to create a communist state in Singapore through bomb attacks, armed revolution and the assasinations of ministers.
Twenty members of a clandestine group called the People's Revolutionary Party of Singapore were arrested, including its leader, Sim Siew Lin.
This was just one among a number of plots to destabilise the newly independent nation after its separation from Malaysia, Singapore Prime Minister Lee Kuan Yew told reporters.
"There are Malay extremist elements, followers of the six wild men in the peninsula, and there are communists who are in league with the Indonesians, who might find it convenient to get rid of my colleagues and me," he said. He did not specify who the "six wild men" were, although he had referred to them as "ultras" or Malay extremists at an earlier press conference.
Maximum security was provided for all Cabinet ministers after Aug 9, and strangers entering City Hall had to be thoroughly screened.
"If I had been knocked down on Aug 8, it would not have been so difficult to keep things as usual as if I were knocked down today or even on Aug 9, after the proclamation," said Mr Lee at a news conference on Aug 26, 1965.
Ringleader Sim Siew Lin was found with correspondence in code with Indonesian intelligence, instructions from Jakarta to prepare a secret entry into Singapore for an important person, and propaganda material against Chinese and Malays here.
The most anxious period was from Aug 9 to 11, he said. "The Indonesians or the six mad mullahs could have started riots and destroyed our image internationally."
There would have been a lot of explaining to do to leaders of the Arab and Muslim states, he added.
The police said they discovered that a group led by Sim had planned to stir discord between the Chinese and Malays, and smuggle in large batches of guns, hand grenades and explosives from Indonesia.
Sim, who had been watched by the Secret Branch, was nabbed after he came out of a meeting with his aides.
He was found with correspondence in code with Indonesian intelligence, instructions from Jakarta to prepare a secret entry into Singapore for an important person, and propaganda material against Chinese and Malays here.
The police said Sim was behind a time bomb placed outside the American Consulate on July 31, 1965. It was neutralised before it could explode. He had been living in hiding in a block of flats at Old Kallang Airport, the police added.
Despite the anxiety over such plots to destabilise Singapore, Mr Lee declared to reporters that the worst had passed: "That anxiety has passed. There will be no race riots in Singapore. Never!"