However, the beleaguered premier said he would stay on and fight for the party.
“Perhaps some other members might leave too, but this won’t entirely affect the party. It all depends on whether the rest want to stay on and continue to fight for the party too,” he said.
Umno Deputy President Datuk Seri Najib Tun Razak expressed his shock and sadness too.
Najib, who read a prepared statement to Malaysian reporters at Sharm El-Sheikh in Egypt, said he was prepared to meet Dr Mahathir to discuss his decision.
"I will also discuss with Datuk Seri Abdullah, whom I had contacted just now, on efforts to strengthen Umno and also the issue of leadership transition in the party.
"I hope all Umno members will stay calm and give support to our efforts to strengthen the party," he added.
In KLANG: Former Selangor Mentri Besar Datuk Seri Dr Mohamad Khir Toyo said Dr Mahathir’s decision will affect public perception of Umno’s strength and possibly discourage youngsters from joining the party.
Many of those aged below 40 years grew up during Dr Mahathir’s premiership and hold him in high regard.
“Dr Mahathir’s resignation will have a huge impact on the trust that this group of people, including the Malays, have in Umno and its capability to lead the country,” he said.
He said youngsters wanting to join the party now might also shy away as questions would be raised on Umno’s ability to be effective as a party.
“These youngsters might not join other parties but they will stay away until Umno can mend the current situation and avoid splits in the party,” he said.
In PUTRAJAYA, Home Minister Datuk Seri Syed Hamid Albar said it was a bad time for Dr Mahathir to quit Umno as the party was going through a difficult period.
He said the resignation would not help in efforts to rebuild Umno.
"He is doing more damage than good; whatever it is, the party is still looking for a new direction and we need everyone to remain united and not split," he said.
Syed Hamid expressed fear for the future of Umno because Dr Mahathir had a great influence and a person of his stature could rattle and destroy the party.
Domestic Trade and Consumer Affairs Minister Datuk Shahrir Abdul Samad said Dr Mahathir's hatred for Abdullah was seemingly stronger than his love for Umno.
He said the former premier should realise that asking others to follow his footsteps would only make it easier for Pakatan Rakyat to take over the Federal Government.
"What he is saying is that if you love Umno, leave Umno -- it does not make sense," he said.
Shahrir also said that Jerlun MP Datuk Mukhriz Mahathir should be the first to take up his father's suggestion to quit.
Umno information chief Tan Sri Muhammad Muhammad Taib said he was surprised at the decision, but added that it was Dr Mahathir's right to do so.
"Every member has the right to join or leave the party," he said.
Asked if Umno would convene an emergency meeting, the former Umno vice-president and Selangor Mentri Besar said that "we will wait and see."
The next Umno Supreme Council meeting is scheduled to be held within two to three weeks.