Prime Minister Datuk Seri Najib Razak said police should investigate an incident that occurred on the sidelines of the 'Red Shirt' rally today.

Najib, however, noted that the event, formally called 'Himpunan Rakyat Bersatu' (United People's Gathering) at Padang Merbok here, took place peacefully.

"(I) was informed that the rally in KL ended peacefully except for an incident. PDRM (police) should investigate," he said on his Twitter account.

While the rally went on smoothly at Padang Merbok, a group was causing trouble in Petaling Street, trying to breach the police barricade around the popular tourist destination.

Stating that it should not have happened, the prime minister said everyone must abide by instructions from the authorities.

Rally organisers led by the National Silat Federation (Pesaka) disassociated themselves from the group.

Today's rally was held in reaction to the Bersih 4 street demonstration late last month.