* Opposition decries violent tactics of Right Sector
* But the group has re-energised the protests against Yanukovich
* One estimate says has 300 members on street, supported by donations
* Right Sector social network page has more than 100,000 followers
* Insiders say its origins are among nationalist soccer fans
By Pavel Polityuk and Richard Balmforth
KIEV, Jan 24 (Reuters) - To Ukraine's embattled authorities they are terrorists and fanatics. To opposition leaders striving to keep President Viktor Yanukovich and his government under pressure by peaceful means they are a useful weapon but a dangerous liability.
Whether either side likes it or not, it is the violent tactics of a determined, far-right group called "Right Sector" that is setting the agenda on the streets of Kiev.
Insiders say the group has its origins among nationalist-minded soccer fans - the word 'sector' in Russian denotes the spectator terraces of a stadium - and includes individuals from far-right organisations from across the country.
They attack the police, sometimes in pairs, usually wearing masks or helmets and armed with sticks, iron bars and 'trophy' shields seized from riot police.
They have no allegiance to any established political party and operate under no banner, flag or standard, not even that of recognised nationalist groups.
Their numbers are unknown - though some say they have 300 people on the streets of Kiev - and they draw support from across Ukraine, with more than 100,000 supporters on Russian-language social network Vkontakte.
In its mission statement on Vkontakte, Right Sector says it aims not only to force Yanukovich to sign the European Union free-trade agreement he walked away from in November, setting off the protests, but by "revolutionary" means to get rid of his "regime of occupation".
Its members, nearly all male, though a few women have been seen among them, have turned their backs on the peaceful protest advocated by the mainstream opposition and have kept riot police on the back foot with a hail of petrol bombs and cobblestones for the past week.
By their violent tactics, they have hijacked the protest movement, known as Euromaidan - literally Eurosquare - and added a combustible element that both the authorities and the opposition are struggling to contend with.
Opposition leaders disown the violence, recognising that it gives the authorities the perfect excuse for a crackdown that might snuff out the protest movement.
But Right Sector has also re-energised the movement and raised the heat on Yanukovich, serving a warning of what lies ahead if he won't compromise.
When the opposition emerged empty-handed from talks with Yanukovich late on Thursday, it was masked Right Sector members who began immediately overseeing the building of new barricades.
The focus of protest has shifted from Kiev's Independence Square, the crucible of the Euromaidan movement, to a kilometre away, where Right Sector warriors and others clash daily with police, down the road from the main government building.
The area near soccer club Dynamo Kiev's colonnaded stadium has now become their theatre of operations, a battlezone where protesters confront black-helmeted riot police across a 40-metre no-man's land.
On Thursday, protesters demonstrated there in front of a curtain of billowing black smoke from burning tyres, obscuring police lines, to a drum beat of sticks on corrugated metal.
"It's a self-organising structure with links with all right-wing organisations which are ready to defend the interests of the Ukrainian nation, the Ukrainian idea," said a man who gave only his nickname, Zaliznyak (Ironman), and described himself as the military chaplain of the group.
"Many people regard it as pointless to stand around on the Square listening to leaders who say our weapons are our torches," he told Reuters, referring to the protesters' practice of lighting up their mobile phones to show resistance.
Zaliznyak, in his 50s, with greying hair and a penetrating gaze, says the heavy-handed force shown by the riot police justifies their actions.
"When they shoot at people, shoot at children, when they march across the bellies of pregnant women, then it is clear that any normal person must defend the weak," he said on the fifth floor of a building on Independence Square, one of the group's hang-outs.
Despite their mission statement, they have little political agenda as such, insiders say. Unlike most of the mainstream protesters, they do not want EU membership, though they oppose close association with Russia as a threat to independence.
Supporters - particularly the younger among them - speak in romantic terms of patriotic Robin Hood figures, righting wrongs in Ukrainian society.
"They are always on the front line against the Berkut (riot police), who are afraid of them because they are capable of anything," said 21-year-old Serhiy, a student protester, who says he is not a member but knows their ways.
"There are up to 300 people in the organisation. They train for their actions and they are indeed better than us. They train and send really experienced people. They only have one aim - 'Ukraine before all'."
"There is no centralisation in the organisation. There are no lists and their members operate under complete anonymity. Their formation represents a spontaneous reaction to the lawlessness of today's authorities," said 33-year-old Vasil from Volyn in western Ukraine, who says he has got to know some Right Sector members from chatting to them on the front line.
He said they were supported by donations from the public; one of their girlfriends managed to collect about $1,000 in a day.
The challenge for the opposition leaders is how they can continue to benefit from the tactics of Right Sector without condoning them and alienating moderate supporters, many of whom are dismayed by the street violence.
Zaliznyak is dismissive of the opposition leaders - chief among them boxer-turned-politician Vitaly Klitschko, former economy minister Arseny Yatsenyuk and far-right nationalist Oleh Tyahnibok.
"There can be only one leader, and at the moment there is a three-headed leadership," he said. "A person whom we do not know yet will come forward as a leader," he added.
When Klitschko tried on Sunday to stop people attacking riot police, one protestor set off a fire extinguisher in his face.
But immediately after ending fruitless talks with Yanukovich on Thursday night, Klitschko and Tyahnibok went first to the barricades at Dynamo stadium to give an account of their meeting and appeal for protesters to extend a truce.
Tyres were ablaze again at the site on Friday, suggesting their words were only partially heeded.
"The uncontrolled radicalisation of the protests is a problem. The danger is that Right Sector has no clear aims other than struggle and clashes. The opposition is finding it more and more difficult not only to control them but to keep them in check," said Volodymyr Fesenko, a political analyst. (Writing By Richard Balmforth; Editing by Will Waterman)