Mr Low Thia Khiang, 63, former secretary-general of the Workers' Party (WP), is in the intensive care unit of a hospital, after suffering a head injury from a fall at home on Thursday.

The party said in a media statement yesterday that the Aljunied GRC MP is conscious.

"He is being monitored in the intensive care unit. Mr Low is conscious. Mr Low's family has requested for privacy so he can focus on his recuperation."

In Mr Low's absence from work, the other Aljunied GRC MPs will cover his constituency duties, assisted by former Non-Constituency MP Gerald Giam.

Mr Low entered politics in 1988, losing in his first outing as a WP candidate in Tiong Bahru GRC.

He became MP after winning the Hougang seat at his second election in 1991, and is now the longest-serving opposition MP.

Mr Low took over as secretary-general of the WP from Mr J.B. Jeyaretnam in 2001 and went on to lead the party for 17 years.

He announced in 2017 that he would step down in a year.

He said he was leaving the party's top post after achieving the two goals he had set for himself - electoral success and party renewal.

Mr Pritam Singh was elected secretary-general in 2018 after Mr Low stepped down.

Mr Low is credited with being the first opposition leader to win a group representation constituency when the WP team he led won Aljunied GRC in 2011.

It retained the constituency in 2015, though there had been talk that Mr Low was considering a run in a different constituency at the next election.

As news spread of Mr Low's fall yesterday, people from across the political spectrum sent wishes for his recovery.

Deputy Prime Minister Heng Swee Keat wished Mr Low a speedy recovery and said he was "hoping to see him back to Parliament soon".

Senior Minister Tharman Shanmugaratnam said he very much hoped Mr Low "recovers well and gets back to his vigorous self".

Similarly, Trade and Industry Minister Chan Chun Sing said: "I wish Mr Low a quick recovery. I just had a good, long conversation with him at the last Parliament sitting."

Speaker of Parliament Tan Chuan-Jin wrote: "I knew he had a fall when Pritam applied for leave from Parliament sitting for him but I didn't know that it was quite so serious. Our thoughts and prayers are with you and your family, Mr Low. Wishing you speedy recovery."

Tampines GRC MP Cheng Li Hui said she had hoped to give Mr Low matching masks for him and his grandson at the parliamentary sitting today.

"Sorry to hear about Mr Low's fall. Was going to give him grandpa-grandson matching blue masks... Met the adorable boy at NDP last year. Only grandchildren can make grandparents smile so sweetly," she said.

The Singapore Democratic Party also wished Mr Low a quick recovery, while Mr Goh Meng Seng, who leads People's Power Party, said Mr Low was his first political mentor and he owed a lot of his political foundation to Mr Low.

"I hope he could have a speedy recovery," he said.

Dr Tan Cheng Bock, who leads the Progress Singapore Party, said: "Praying that he recovers quickly and is back to full health soon."