Thai riot police have fired tear gas to control an anti-government crowd that has massed in downtown Bangkok.

Thousands of police were deployed for the rally Saturday, organized by the royalist group Pitak Siam, which wants Prime Minister Yingluck Shinawatra's government to step down.

The government has invoked the Internal Security Act, allowing police to detain protesters.

Authorities say police fired tear gas at a group of demonstrators who tried to break through barriers and were not following police orders.

Several police officers and protestors have been injured in clashes.

The new Pitak Siam group, led by retired general Boonlert Daewprasit, accuses Prime Minister Yingluck's Puea Thai party of corruption and being a puppet of former prime minister Thaksin Shinawatra. He was ousted in a 2006 military-backed coup and fled the country in 2008, shortly before being found guilty of abuse of power.