KUALA LUMPUR - A demonstration calling for the arrest of Prime Minister Najib Razak over graft claims fizzled out in under an hour on Saturday after police arrested at least 20 protesters including two opposition state assemblymen.

Nearly 100 people gathered outside the popular Sogo shopping mall in Kuala Lumpur in the aftenoon, despite police declaring the rally organised by youth activist group Demi Malaysia as illegal and arresting four of the organisers ahead of the gathering.

The crowd became rowdy after police arrested Teja and Simpang Pulai assemblymen Chang Lih Kang and Tan Kar Hing.

Those chanting "Tangkap Najib (arrest Najib)" and "Hidup Rakyat (long live the people)" and throwing fireworks were quickly hauled away by police, including at least two elderly protesters.

But the crowd ran helter skelter and regrouped several times. Police were then seen dragging away protesters, some of whom were tackled to the ground, before being loaded into police trucks and brought to a nearby police station.

One protester, Azira Aziz, tweeted from the police station that some protesters suffered injuries in the melee, but were treated well by police and were given medical treatment and water.

At 3pm, police, using loud hailers, ordered the public to leave the area after Sogo and surrounding shops were shuttered.

The student activists had organised the rally to call for the arrest of Datuk Seri Najib, who was accused by The Wall Street Journal on July 3 of receiving US$700 million (S$958 million) linked to troubled state investor 1Malaysia Development Berhad (1MDB) in his personal accounts over the past two years.

The prime minister has denied ever using state funds for personal gain but failed to clarify if the money did indeed enter his bank accounts.