Former president S R Nathan, 92, is in critical condition after he had a stroke early yesterday morning.
He remains in the intensive care unit at Singapore General Hospital, the Prime Minister's Office said in a statement.
Prime Minister Lee Hsien Loong, who was on his way to Washington, DC for an official visit to the United States, said he was saddened to learn about Mr Nathan's stroke and his grave condition, and had been in touch with his family.
"Mr Nathan served as our Ambassador to the US in the 1990s, among many contributions in a long career," Mr Lee said in a Facebook post. "We have him in our thoughts and prayers."
Singapore's sixth and longest- serving president, Mr Nathan was head of state from 1999 to 2011.
He had been unwell recently, and had a stroke in April last year that saw him hospitalised for almost three weeks.
A number of Mr Nathan's friends and former colleagues visited him yesterday afternoon.
PRAYING FOR HIM
Mr Nathan served as our Ambassador to the US in the 1990s, among many contributions in a long career. We have him in our thoughts and prayers.
PRIME MINISTER LEE HSIEN LOONG, in a Facebook post. Mr Lee, who was on his way to Washington, DC for an official visit to the United States, said he was saddened to learn about Mr Nathan's grave condition, and had been in touch with his family.
President Tony Tan Keng Yam and his wife, Mrs Mary Tan, visited for about half an hour.
Dr Tan, who saw Mr Nathan and spoke with his family, said he and his wife were sad to hear of Mr Nathan's hospitalisation.
He also said in a Facebook post: "We will continue to keep Mr Nathan in our prayers."
Other visitors included Acting Prime Minister Teo Chee Hean, Chief Justice Sundaresh Menon, and former Speaker of Parliament Abdullah Tarmugi.
Emeritus Senior Minister Goh Chok Tong, an MP for Marine Parade GRC where Mr Nathan lives, also visited him in hospital and said that according to the doctor, his vital signs were stable, but it was too early to say how he would progress.
"He's a fighter," Mr Goh added.
Mr Nathan had a distinguished career in public service spanning labour relations, security and diplomacy. He joined the National Trades Union Congress' Labour Research Unit in 1962, handling negotiations between unions and employers.
He later led the Security and Intelligence Division, and put his life on the line when he accompanied hijackers of the ferry Laju to Kuwait to guarantee their safe passage.
He also served as executive chairman of Straits Times Press and High Commissioner to Malaysia.
He was Singapore's second elected president from 1999 to 2011, and stayed active after retirement, publishing his memoirs and spearheading several books.
He also met young Singaporeans to share his thoughts on how they have to ensure Singapore stays successful.