Activists have also said they will try to disrupt an annual flag-raising ceremony attended by senior officials.
Hong Kong: Thousands of people rallied in support of the Hong Kong police on Sunday as the semi-autonomous territory readied for another day of protests on the anniversary of the former British colony's return to China.
A large crowd filled a park in front of the legislature and chanted "Thank you" to the police, who have been criticized for using tear gas and rubber bullets during clashes with demonstrators that left dozens injured on June 12.
A protest march has been called for Monday, the third in three weeks, this one on the 22nd anniversary of the handover of Hong Kong to China on July 1, 1997.
Activists have also said they will try to disrupt an annual flag-raising ceremony attended by senior Hong Kong and mainland Chinese officials in the morning.
Police have erected tall barriers and shut off access to Golden Bauhinia Square, where the flag-raising will be held, to prevent protesters from massing there overnight.
The anniversary always draws protests, but this year's is expected to be larger than usual because of widespread opposition to a government proposal to allow suspects to be extradited to mainland China to face charges.
More than a million people took to the streets in two previous marches in June, organizers estimate.
The government has already postponed debate on the extradition bill indefinitely, leaving it to die, but protest leaders want the legislation formally withdrawn and the resignation of Hong Kong's leader, Chief Executive Carrie Lam. They also are demanding an independent inquiry into police actions on June 12.