HONG KONG — Black-clad demonstrators rallied in Hong Kong’s airport on Friday, filling the arrivals hall of one of the world’s busiest terminals as the city braced for another weekend of potentially combustible protests.

Activists also signaled that despite objections from the police, they would continue with plans for a Saturday rally against mob violence in Yuen Long, a district near the mainland Chinese border where last weekend a group of men attacked people in a train station and on nearby streets.

That attack on Sunday, which left at least 45 people injured, was apparently meant to intimidate the protesters who have been holding demonstrations in the city for weeks. But the men, many of whom were masked and dressed in white T-shirts, also lashed out at train passengers who had no apparent connection to the demonstrations.

The police — who failed to stop the mob, and initially made no arrests — have since detained 12 people in connection with the train station attacks, including some accused of having connections to the criminal gangs known as triads. The authorities have said they object to the Yuen Long rally on Saturday because of the risk of clashes, with tensions running high between pro-democracy protesters and residents of the district’s villages, who are more conservative and supportive of the establishment.