Pro-democracy protesters rally on a major road in the heart of Hong Kong near the government headquarters on September 28, 2014. Police repeatedly fired tear gas after tens of thousands of pro-democracy demonstrators brought parts of central Hong Kong to a standstill on September 28, a dramatic escalation of protests that have gripped the city for days. AFP PHOTO / ALEX OGLE (Photo credit should read Alex Ogle/AFP/Getty Images)

BEIJING -- Pro-democracy protests in Hong Kong reached a boiling point on Sunday, with police employing tear gas in attempts to control crowds that have surrounded government buildings for the past three days.

Tension has been escalating in the city all summer, after Beijing released a controversial plan for Hong Kong's 2017 elections that activists say falls short of the full democracy they hope for. The plan would allow for universal suffrage in three year's time, but would restrict candidates to those approved by a committee that opposition politicians say is dominated by pro-Beijing representatives.

In response, pro-democracy groups Occupy Central and student-led Scholarism have threatened civil disobedience that could shut down the city. After a week of student boycotts and protests, the confrontation escalated on Sunday when police began using heavy-handed tactics to dispel demonstrators. Police have been using pepper spray and tear gas, but protesters continued to pour into the streets on Sunday night:

Tear gas and chaos: The images coming from Hong Kong are insane http://t.co/2b6hrcYrKxpic.twitter.com/GCQKD7t5C0 — Mil/Def Insider (@MilDefInsider) September 28, 2014

LIVE: Police struggling to contain masses of angry protesters #OccupyCentralpic.twitter.com/Jlvt1qKIyi — SCMP News (@SCMPNews) September 28, 2014

Police have been photographed carrying guns, and some protest groups called for a retreat on Sunday night on fears that the police would begin using rubber bullets.

I just stumbled onto this photo. It speaks for itself. #OccupyCentralpic.twitter.com/IBVOm55xKx — Andrew Peng (@TheAPJournalist) September 28, 2014

Photos posted online showed subways full of police being transported toward the protests. In response, protestors barricaded the exits to some subway stations.

Students trying to block MTR exit on rumors that lots of police are transported and entered Admiralty via subway. pic.twitter.com/d660x6m8vm — Fiona Law (@law_fiona) September 28, 2014

Students prepared for the threat of tear gas with masks, and as the night wore on, protestors issued public appeals for more supplies on social media.

"In clash, raise your hands to show you don't want to attack," #OccupyCentral protesters told http://t.co/6wAn5eTNmVpic.twitter.com/OS1NRnmnTs — SCMP News (@SCMPNews) September 28, 2014

Police reportedly cleared the site of initial protests, but stand-offs continued at the government headquarters and other locations near the center of the city.

Here's the original protest area. Quite empty after police seal-off, as protesters captured nearby highway instead. pic.twitter.com/vUBzdT85Jv — Alan Wong (@alanwongw) September 28, 2014

Outside the Hong Kong club building, 15 riot vans, cops with batons out #hk926#OccupyCentralpic.twitter.com/ch39W1pVua — Benjamin Haas (@haasbenjamin) September 28, 2014

At approximately 10:30 p.m., a Twitter account for the Hong Kong Federation of Students, one of the leading protest groups, began to call for a retreat in the face of mounting injuries.

We are urging protesters to retreat. Stay safe. This is a long battle #hkclassboybott#OccupyCentral — 學聯 HKFS (@HKFS1958) September 28, 2014