SINGAPORE - Prime Minister Lee Hsien Loong sent a letter of condolence to the wife of late President S R Nathan on Tuesday (Aug 23).

"Please accept my deepest condolences on the passing of your husband S R Nathan," Mr Lee wrote.

Mr Nathan's life is an inspiration to everyone, said Mr Lee. Mr Nathan, who died on Monday at age 92, had a difficult childhood but rose through the ranks of public service and came to serve as Singapore's President for 12 years.

"It was a story of how Mr Nathan always put his nation before self, often at great personal sacrifice," wrote Mr Lee.

Mr Lee said that Mr Nathan had a varied and stellar public career, praising his "savvy judgment and moral courage".

He "played an instrumental role in preventing the Communists from taking over Singapore", he wrote.

At the Ministry of Foreign Affairs (MFA), which Mr Nathan joined shortly after Singapore's independence in 1965, he "helped build up our diplomatic networks", wrote Mr Lee.

Mr Nathan also served at the Security and Intelligence Division before returning to the MFA as permanent secretary.

Mr Lee added that Mr Nathan served as Singapore's longest-serving President with "dignity and distinction".

"He impressed visitors with his knowledge of world affairs," Mr Lee wrote. "He was a warm and approachable President who endeared himself to Singaporeans."

He established the President's Challenge to help the less fortunate.

After stepping down as President in 2011, he joined the Institute of South-east Asian Studies and the S Rajaratnam School of International Studies as a Distinguished Senior Fellow.

There, he continued to share his experiences and insights with a new generation of diplomats, students and Singaporeans, wrote Mr Lee.

"I have known Mr Nathan for almost 40 years. I remember him as a man who lived his life guided by a deep sense of duty to the nation," Mr Lee wrote. "Without fail, he stepped up each time. He was a true son of Singapore."

Around noon on Tuesday, PM Lee and Mrs Lee attended the private wake of Mr Nathan at the family house in Ceylon Road.