Prime Minister Datuk Seri Najib Tun Razak today described Tun Dr Mahathir Mohamad's actions in betraying UMNO and working with the Opposition as an act of treason at its peak.

He said Dr Mahathir who was previously a well-respected statesman, loved by all, has now turned against the party which he once led for years.

"Dr Mahathir is now involved in despicable acts, conspiring with the DAP, the Bersih illegal assembly, Parti Amanah Negara and PKR.

"What's worse the former national leader is also conspiring with foreign enemies and currency crook, George Soros who he onced hated. And this is the peak of his treason to UMNO, our race and our country," said Datuk Seri Najib in his presidential policy speech at the 2016 UMNO General Assembly at the Putra World Trade Centre, here today.

Datuk Seri Najib also said he was disappointed by Dr Mahathir who betrayed the party to be with the Opposition, and even seek aid from billionaire George Soros - a person whom the former premier once strongly criticised.

Meanwhile, Datuk Seri Najib thanked UMNO deputy president Datuk Seri Dr Ahmad Zahid Hamidi and vice president Datuk Seri Hishammuddin Tun Hussein for being loyal to the party and its leadership.

He is also confident that the Pagoh UMNO division and Semporna UMNO division would remain loyal as well.

The opening of 2016 UMNO General Assembly began with a parade and hoisting of the UMNO flag at Plaza Merdeka, the Putra World Trade Centre (PWTC), here today.

Datuk Seri Najib Tun Razak who is also UMNO president arrived at 9am with his wife Datin Seri Rosmah Mansor and were received by Wanita UMNO chief Tan Sri Shahrizat Abdul Jalil, Youth chief Khairy Jamaluddin and Puteri chief Datuk Mas Ermieyati Samsudin.