The months-long stand off in Ukraine involves a number of groups which espouse direct action and often find themselves at the extreme end of the violence that has marred the crisis.
They include both opposition factions and those who support President Viktor Yanukovych.
Anti-government groups often appear to act independently of the more established three opposition leaders who represent parliamentary opposition parties (Udar, Fatherland, Freedom).
There have been allegations that various pro-government vigilante groups have received support from government officials and the police.
The Right Sector is a radical nationalist opposition group said to be a key factor behind the recent violence in Kiev.
The group emerged in the media spotlight after its activists clashed with police in central Kiev on 19 January. The group's account on the VKontakte social network that day encouraged members to come to the scene, bringing bottles for Molotov cocktails and bombs.
According to one Right Sector leader, Andriy Tarasenko, the organisation was set up in late November 2013 and includes several small ultra-right groups, among them Trident, Patriot of Ukraine, White Hammer and Ukrainian National Assembly-Ukrainian National Self-Defence. "But most participants are just ordinary citizens having no relation to any organisations," he said.
Although Right Sector activists have been participating in the pro-EU protests sparked by President Viktor Yanukovych's sudden U-turn on EU integration from the start, their ultimate goal, Mr Tarasenko said, is not having closer ties with Brussels but "staging a nationalist revolution".
Together with the Right Sector, Common Cause is also at the extreme end of the Ukrainian protest movement, though it does not appear as yet to share the former's relish for street fighting.
It is best known for capturing several key government offices in Kiev, such as the ministries of justice, agriculture, and energy.
The group has called for early parliamentary and presidential elections, and describes any opposition leaders who may urge protesters to disperse before the early polls "either idiots or provocateurs".
"If we don't force the authorities to go today, we'll regret it tomorrow," says the group's website.
Common Cause's leader is Oleksandr Danylyuk, a lawyer and a rights activist who was earlier involved in campaigns against former President Leonid Kuchma and later Kiev Mayor Leonid Chernovetskyy.
The capture of ministries caused deep rifts within the opposition, especially after Justice Minister Olena Lukash threatened to declare a state of emergency unless her ministry was vacated.
More moderate opposition leaders have accused Mr Danylyuk and his movement of staging "provocations" which can undermine talks with the government and serve as a pretext for imposing martial law.
The activists left the agriculture ministry under physical pressure from Freedom party members, reportedly after shots were fired and at least three people wounded.
Earlier, Freedom activists reportedly forced Common Cause out of the Kiev city administration building - captured by the opposition earlier.
Common Cause held the energy ministry for a few hours on 25 January, purportedly just to show that it could take control of the building if it wanted.
Pro-government vigilantes, known as "titushkos", have been highly visible in Ukrainian politics in the past year.
They are named after Vadym Titushko, a martial arts enthusiast who was filmed assaulting a female journalist and her photographer husband after a pro-government rally in central Kiev in May and was later prosecuted following an outcry from the local media community and politicians.
"Titushkos", infamous for their aggressive behaviour, are generally thought to be hired in predominantly Russian-speaking parts of eastern and central Ukraine, where support for the government is stronger.
Opposition activists say that they are then bussed in to Kiev. There have been numerous reports accusing them of assaulting demonstrators and journalists alike, often with apparent police connivance.
Dnipropetrovsk Governor Dmytro Kolesnikov later called the people inside the administration offices "representatives of the region's public organisations", adding: "Everything is perfectly legal."
As mass rallies evolved into violent clashes across Ukraine in late January, many football fan groups announced their support for the anti-government demonstrations and took to the streets to protect the protesters from attacks by vigilantes.
Hardcore football fans in Ukraine, known as "ultras", often have far-right leanings and a historically antagonistic relationship with police.
The first group to announce its intention to protect Kiev residents from "titushkos" was the fan club of Dynamo Kiev FC on 21 January.
Two days later, the ultras of Dynamo's arch rival and reigning Ukraine champion, Shakhtar Donetsk, came out to protect anti-government protesters in Donetsk, the stronghold of President Viktor Yanukovych's ruling Party of Regions.
Fans in eastern and central regions, which are known to be sceptical about the anti-government rallies, quickly followed suit.
Fan groups of at least 16 clubs have announced their support for anti-government protests and pledged to protect the demonstrators.
Oplot, or "Stronghold" in Russian, is a pro-government group based around a sports club in the eastern city of Kharkiv that describes itself as "the first fight club in Ukraine".
It is led by Yevhen Zhilin, a retired police captain who champions the Soviet Union's legacy, especially its military heritage.
Mr Zhilin and members of his club are fiercely opposed to the anti-government protests and say they have visited Kiev to "help police restore order".
The group's exploits include the blocking of activists from Auto-Maidan, an anti-government movement which involves motorists who use their cars to pick up demonstrators, picket properties belonging to government officials and block streets, preventing police deployment to protest sites.
More recently accusations have appeared in social media that Oplot was involved in the kidnapping and torture of Auto-Maidan leader Dmytro Bulatov.
They are based on a report issued by the group on 22 January, shortly before Mr Bulatov went missing. It says that in a scuffle with Oplot fighters in Kiev an unnamed opposition activist has had his ear cut off - an injury also inflicted on Mr Bulatov.
Earlier, in April 2013, Mr Zhilin turned up at an opposition rally in Kiev, where he challenged opponents to a "cage fight with no rules".
Even though Oplot fighters have been accused of doing the bidding of Kharkiv's pro-Russian mayor Hennadiy Kernes, Mr Zhilin spent a total of two years and nine months in detention in 2005-07 on charges of blowing up his car. Mr Zhilin says he was later acquitted of the charges.