Some reports suggested police charged after the demonstrators had breached their barriers, sparking scuffles that caused minor injuries on both sides.
Protesters on social media accused the police of an unprovoked attack.
Leaders on both sides have called for calm, and confirmed that talks between protest leaders and the government delegates will take place on October 21.
The protesters, many of them youths and students, are angry at China’s rulers for limiting their choice of leader in the next election in 2017.
They accuse Hong Kong’s current leader, CY Leung, of failing to stand up to the Chinese Communist Party.
Protests erupted last month and have been going on intermittently around government buildings and the business district on Hong Kong island, and in Mong Kok, a residential and shopping area in Kowloon.
On October 17, police had all but cleared the Mong Kok site and protesters’ numbers had substantially dwindled elsewhere.
Clashes resumed later as protesters launched a sustained effort to reoccupy a busy road junction in Mong Kok.
About 9,000 protesters pushed police back, with 26 people arrested.
At about midnight on October 18, police charged protesters, beating them with batons and deploying pepper spray.
A stand-off resumed shortly afterwards with neither side having gained any ground.
Talks between the two sides were announced earlier.
Carrie Lam, CY Leung’s deputy, said both sides would send five representatives to the negotiations, which will be broadcast live on television.
The talks will last about two hours, and be focused on constitutional reform, Carrie Lam said.
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