HONG KONG—Police arrested and charged a dozen protesters suspected of involvement in antigovernment clashes and widened a hunt for others who joined a rampage through the city’s legislature.

The arrests signal that the government is pressing a newfound advantage, after it was caught off-guard by a wave of popular rallies that scotched its attempt to drive through an unpopular extradition law. Officials were forced to suspend the bill, which would have moved the semiautonomous city closer within Beijing’s legal jurisdiction....