A Chinese state-run newspaper has blamed the United States for being behind the pro-democracy protests that have rattled Hong Kong -- a claim the U.S. State Department strongly rejected.

Thousands of people, meanwhile, returned for sit-ins in Hong Kong's main protest zone Saturday, responding to organizers' calls to boost a civil disobedience campaign that has paralyzed key roads and streets in the city center for two weeks.

Students and activists leading the protests remain locked in a stalemate with the government, which has called off scheduled negotiations and instead urged protesters to retreat from the streets. Protest leaders have vowed to keep up the demonstrations until the government responds to demands for voters to have a greater say in choosing Hong Kong's leader.

In a commentary published on the front page of the Communist Party-run People's Daily's overseas edition Friday, the newspaper said the National Endowment for Democracy, a Washington, D.C.-based nonprofit group, became involved in the Hong Kong protests as part of a U.S. strategy to undermine foreign governments in the name of promoting democracy.

Citing unidentified media reports, the commentary claimed that Louisa Greve, a director at NED, met with Hong Kong protest leaders months ago to discuss the movement.

The group did not immediately reply to an email requesting comment Saturday. According to its website, the organization is devoted to "the growth and strengthening of democratic institutions around the world," and is funded largely by the U.S. Congress.

When asked about the U.S. State Department's role in the Hong Kong protests, department spokeswoman Marie Harf said Friday that U.S. officials "categorically reject accusations that we are manipulating the activities of any person, group or political party in Hong Kong."

"What is happening there is about the people of Hong Kong, and any assertion otherwise is an attempt to distract from the issue at hand, which is the people expressing their desire for universal suffrage in an election that provides a meaningful choice of candidates representative of their own voters' will," Harf said.