HONG KONG — Pro-democracy demonstrations in two of Hong Kong’s most crowded shopping districts came under attack on Friday from unidentified men who assaulted protesters and tore down their encampments, after a week of erratic and unsuccessful attempts by the Beijing-backed government to end the protests.

The protesters said the attackers were pro-government gangs, and several protest groups called off planned negotiations with the government in response.

Crowds of residents, fed up with the inconveniences of the protests, had cheered on the attacks on the camps.

On Saturday morning, a police spokesman said 19 men, including eight with links to organized crime syndicates, or triads, had been arrested in connection with the violence in one of the districts, Mong Kok, according to Radio Television Hong Kong.