In a statement on Facebook, the police said the girl was running in a “suspicious manner” that required officers to chase and subdue her with the use of “minimum force”.
The arrest came amid demonstrations against delayed elections, with hundreds of protesters facing off against thousands of riot police.
About 290 people were arrested in the biggest, single-day sweep since early July, in one of the largest demonstrations since a draconian national security law was imposed by Beijing on the territory.
The new law, introduced on July 1, criminalises acts deemed as secession, subversion, terrorism and foreign collusion, making them punishable by life imprisonment.
Protest slogans and songs are banned, and books deemed politically sensitive have been pulled from public library shelves.
During the protests, activists were arrested for a range of alleged infractions, including chanting anti-government protest slogans, engaging in unlawful assembly, misconduct in a public place, and failing to produce identification.
Before the protest, activist Tam -Tak-chi, 47, was arrested on suspicion of “uttering seditious words”, police said. Pro-democracy activist Joshua Wong appeared near the starting point of the march early in the afternoon, but it is unclear if he managed to join it due to the heavy police presence.
“We hope the world can never forget September 6 should be election day. And now Beijing delayed and even cancelled the elections, which is totally unreasonable,” he said.
Clashes erupted on multiple occasions as police fired rounds of pepper balls while protesters hurled water bottles and an umbrella towards them.
The 2020 Hong Kong Legislative Council election was originally scheduled for last Sunday, until it was postponed for a year by the government, citing coronavirus concerns. Activists have criticised the city’s authorities for using the pandemic to delay an election it appeared poised to lose, especially as the decision to suspend the polls came after several pro-democracy opposition candidates were barred from running.
“The government took away our voting rights,” Wu, 23, a clerk, said.
In recent months, authorities have engaged in a wider crackdown on dissent, which has included the mass arrest of leading pro-democracy activists. Several have fled overseas, including to Britain. Journalists, academics and politicians were also arrested.