* Pro-democracy protesters say clashes co-ordinated
* Anti-protest groups vow to return
* Police restore uneasy calm
* Some barricades in Admiralty district fortified (Adds fortification of barricades, fresh quotes)
By Yimou Lee and Diana Chan
HONG KONG, Oct 13 (Reuters) - Hundreds of people tore down protest barriers in the heart of Hong Kong's business district on Monday, scuffling with pro-democracy demonstrators who have occupied the streets for two weeks.
Police intervened to separate the rival groups, and calm was restored. In an indication that the crisis would not be over soon, however, protesters fortified blockades on the perimeter of the downtown district of Admiralty.
Bamboo scaffolding four meters high was erected along one major thoroughfare, while groups of demonstrators mixed concrete to pour over the foundations of their road blocks.
"This shows our determination to fight for democracy," said Bert Tseng, a university student who has been at the protests for the past two weeks.
"We've now reached a stage where even if they dismantle everything and clear the roads, we'll just regroup and do it somewhere else. They can't demolish our spirit."
Earlier in the day, angry taxi drivers opposed to protests which have seriously affected their business had rallied at one barricaded road, demanding an end to the disruptions.
"Open the roads!" chanted a crowd, which included taxi and truck drivers. Cab drivers have given protesters a deadline of Wednesday evening for all barricades to be lifted.
A truck with a crane on top attempted to remove barricades from one area until police eventually stopped it, but protesters complained that police did not act quickly enough.
Hundreds of police had earlier dismantled some barricades to relieve traffic congestion in the Asian financial hub, but said protesters could remain.
RIVAL GROUPS CONVERGE
Within hours, anti-Occupy Central groups descended on the protest sites to try and disperse demonstrators, taking advantage of the earlier police action to remove barricades.
Scuffles quickly broke out, among the first between demonstrators and anti-protest groups, with protesters believing the attacks were co-ordinated and may have involved triad Asian crime gangs.
Some of the anti-protest groups spoke Mandarin and could not speak Cantonese, suggesting they were not from Hong Kong.
"The group of people who look like gangsters, they start running toward them (protesters), and then one of them hit an old man with something hard, hit his head, so he's injured over there right now," said 20-year-old student protester and witness Winnie Locke.
Eventually police separated the two groups, forming a human barricade between the two, and an uneasy calm returned to the streets, but Hong Kong residents expect more flare-ups.
"We will stay and defend. We will stay here until the end," said 25-year-old pro-democracy protester John, as he repaired damaged barriers.
The protesters, mostly students, are demanding full democracy and have called on the city's embattled leader, Leung Chun-ying, to step down after Beijing in August ruled out free elections for Hong Kong's next leader in 2017.
China rules Hong Kong under a "one country, two systems" formula that accords the former British colony a degree of autonomy and freedoms not enjoyed in mainland China, with universal suffrage set as an eventual goal.
The demonstrations escalated late last month after police used tear gas and batons on demonstrators. Since then, police have been largely hands-off and their presence minimal.
MORE FLARE-UPS EXPECTED
On Monday, police began removing some barricades in the areas of Central and Admiralty, home to global financial institutions and government buildings, as well as the bustling district of Mong Kok, across the harbor from the glittering towers of Hong Kong Island.
Many protesters donned face masks and goggles to protect themselves against possible use of tear gas or pepper spray as police, some carrying small riot shields, moved in.
Tensions escalated near the main protest site after noon as nearly 200 mostly elderly, pro-Beijing supporters - wearing blue shirt and ribbons - staged a rally as police stood guard.
Many Hong Kong residents and businesses have become frustrated with the inconvenience the protests are causing.
"I used to support the movement, but then my business was affected...what they did is useless anyway," said taxi driver Lee, 25, who estimates he has lost 60 percent of his business.
Lee was part of a crowd of some 15 people with signs saying: "Taxi Drivers & Operators Association: Save livelihood. Can't tolerate anymore." Lee said taxi drivers will return if the protesters do not quit by Wednesday.
The Hong Kong and Beijing governments have called the protests illegal. The Hong Kong government last week called off talks with student leaders.
On Sunday, Leung vowed to remain in office and warned students demanding his resignation that their pro-democracy movement was out of control.
Leung has warned that there was "zero chance" that China's leaders in Beijing would change an August decision limiting democracy in Hong Kong.
The former British colony was promised that its freedoms would be protected when Britain handed its old colony back to China 17 years ago.
Beijing has said that only candidates screened by a nomination committee will be able to contest a full city-wide vote to choose the next chief executive in 2017. (Additional reporting by James Pomfret, Kinling Lo, Bobby Yip, Joseph Campbell and Venus Wu; Writing by John Ruwitch; Editing by Anne Marie Roantree, Michael Perry and Mike Collett-White)