Hong Kong police have clashed with pro-democracy activists overnight, briefly shutting down government offices on Monday.
Protesters fought police armed with pepper spray, batons and water hoses on Lung Wo Road in the Admiralty district.
On Monday, an injunction was granted to clear an area just west of the main Admiralty protest site.
That sparked fears of more violence after what was already some of the worst unrest in two months of protests.
The protesters want the people of Hong Kong to be allowed to choose their leaders in the 2017 elections without intervention from Beijing.
The Chinese government has said it will allow universal suffrage, but candidates for the post of chief executive will be screened first.
After the clashes, Chief Executive CY Leung warned activists not to come back to the protest site later on Monday.
"From this day on, the police will take resolute action when carrying out their duties. I call on the students who are thinking of returning to the occupation area tonight not to do so.
"Don't mistake the police tolerance as weakness."
The unrest flared late on Sunday, after student protest leaders called on supporters to converge on the offices of Mr Leung on Lung Wo Road.
The road is a short distance away from Connaught Road in Admiralty, the major road protesters have been occupying for two months.
Protesters, many wearing hard hats and carrying umbrellas - the symbol of their movement - moved into the area, throwing bottles, helmets and umbrellas towards police.
"I urge everyone to stay here until the morning to keep surrounding the government headquarters. Let's stop the government from functioning tomorrow," a protester in a black T-shirt shouted, Reuters news agency reported.
Police ordered them to retreat, then charged at protesters, eventually forcing them out of the area. Police said that 40 people were arrested and a number of officers were injured.
On Monday morning government offices were shut and staff were told to stay at home. But the roads outside the government site were clear of protesters and open for traffic.
By the afternoon government employees were able to go back to work.
The atmosphere remains tense at Connaught Road where hundreds of protesters are still occupying the area amid a police presence.
Later on Monday, a coach operating company won a court injunction to clear a section of Connaught Road and Harcourt Road, just west of the main Admiralty protest camp.
The site is seen as the major stronghold of protesters and observers say police action to dismantle it could lead to more clashes.
The activists had public support when they started their protest in September, but that is now ebbing as many Hong Kong residents tire of the disruption.
Last week, police dismantled one of the three major protest camps in the Mong Kok commercial district and made 100 arrests.
China has meanwhile hit back at criticism of its decision to bar a UK parliamentary committee from entering Hong Kong, a former British colony.
Foreign ministry spokeswoman Hua Chunying said Beijing had made clear to the UK government many times that it opposed "the so-called delegation of the British parliament's foreign affairs committee going to Hong Kong for a so-called investigation".
She described the MPs' move as "overt confrontation" in a riposte to the committee's chairman, Sir Richard Ottaway, who accused China on Sunday of acting in an "overtly confrontational manner".
Hong Kong democracy timeline
- 1984: Britain and China sign an agreement where Hong Kong is guaranteed "a high degree of autonomy, except in foreign and defence affairs" for 50 years following the handover in 1997.
- June-July 2014: Pro-democracy activists hold an unofficial referendum on political reform and a large rally. This is followed by protests by pro-Beijing activists.
- 31 August 2014: China says it will allow direct elections in 2017, but voters will only be able to choose from a list of pre-approved candidates. Activists stage protests.
- 22 September 2014: Student groups launch a week-long boycott of classes in protest.
- 28 September 2014: Occupy Central and student protests join forces and take over central Hong Kong.
- October 2014: Chief Executive CY Leung dismisses demands for his resignation. Discussions between government and student leaders go nowhere. High court begins granting injunctions to clear protest sites.
- 15 November 2014: Student leaders' attempt to travel to Beijing fails.
- 18 November 2014: Bailiffs move in to clear a portion of the Admiralty protest site.
- 26 November 2014: More than 100 arrests during clearance of Mong Kok site.
- 2017: Direct elections for chief executive due to take place.