Two attempts by Indonesian saboteurs to infiltrate Singapore waters were smashed by Malaysian police over 22 hours on June 25, 1965.
These were the biggest infiltration attempts so far in Indonesia's ongoing Confrontation operation against newly formed Malaysia, which then included Singapore.
Explosions went off around St John's Island as police fired on Indonesian sampans loaded with explosives.
More than 27 Indonesians were involved, with one group aiming to blow up a power station in Pasir Panjang.
In the first incident at around 12.20am, 26 saboteurs from Indonesia tried to sneak into Singapore waters in four sampans carrying explosive devices.
But the Malaysian naval patrols spotted them and went after them.
Three of the sampans blew up. The fourth, with 10 men on board, fled towards Indonesia on being detected.
Seven Indonesians died and nine others were captured.
In the second incident at 9.30pm, a Malaysian patrol vessel rammed an explosives-filled Indonesian sampan at the edge of St John's Island's territorial waters.
The boat exploded and its crew were flung into the sea. Only one body was recovered. Seven people on the Malaysian patrol vessel were also hurt.
The Indonesians involved in the early morning infiltration attempt had been given orders to place bombs at the power station in Pasir Panjang as well as in Katong, Changi and the Malaysian islands off Singapore.
While one group was asked to target the power station, others were briefed to blow up "anything they could find" once they landed on Malaysian soil, said a Malaysian Ministry of Defence spokesman.
"The Indonesians have admitted that they left an Indonesian base at Sekupang, in the Riau Islands, a few miles off Singapore, a few hours before being intercepted by naval patrols," he added.
"They have also admitted that they were completely taken by surprise by the speed with which they were detected so soon after leaving Indonesia."
Drugs were found on one of the captured vessels and several of the men appeared to be in a drugged state. The spokesman said the men had taken drugs "apparently to give them Dutch courage".
Nine Indonesians, aged 15 to 27, were later charged in a Singapore court under the Internal Security Act with illegally entering Singapore waters.