HONG KONG—This city’s government scrapped negotiations with students a day before they were scheduled to take place, a sign the administration is taking a harder line with the protesters, whose numbers have dwindled in the nearly two-week standoff.

The government’s unilateral decision Thursday appeared to double-down on its strategy of letting the protesters continue their sit-ins until they wore out or public opinion turned against them. The move puts pressure on the students to justify their occupation, which has choked traffic and dealt a blow to local businesses.

But student leaders remained defiant, calling on supporters to return to the streets in droves for a large rally on Friday, and joining forces with pro-democracy lawmakers who promised to block legislation in the city’s Legislative Council. Despite the shrinking crowds in recent days, the streets remain barricaded in three key areas of the city.

The government’s pullout was announced by Carrie Lam, the city’s No. 2 official, who is known as a conciliator. The protesters’ main demand is public nomination of candidates for Hong Kong chief executive, the city’s top post. That goes directly against Beijing’s decision on Aug. 31 that the nomination of candidates be approved by a nominating committee largely loyal to Beijing.

Ms. Lam accused the students of “shifting their demands.” She said students were now calling for the Chinese government to rescind its decision, which she called a breach of the Basic Law, Hong Kong’s mini-constitution. She said a previous open letter from students to the government on Oct. 2 didn’t include this demand.