KIEV (REUTERS) - Some wear black balaclavas and grubby army fatigues, others wield spiked iron clubs, all united in a common goal to protect Kiev in the power vacuum following the ouster of president Viktor Yanukovych.

This self-styled defence force also has designs beyond the capital.

In the months of anti-government protests that culminated in fatal gun battles and the fall of the president, a hard core of demonstrators has coalesced into units of about 100 men, forming the vanguard of the fight to bring down Yanukovych and recalibrate Ukraine towards Europe.

It was these groups, known as 'sotnyi', that took control of Yanukovich's offices after he fled the capital late on Friday.

Their men continue to patrol the streets, standing guard outside state buildings or marching two-by-two through the crowds on Kiev's Independence Square, widely known as Maidan and the crucible of an East-West tug-of-war over this country of 46 million.

On Institutska street, the site of some of the bloodiest fighting of last week's clashes, two flak-jacketed "defenders of Maidan" controlled the entrance to Ukraine's Central Bank, a central pillar of its teetering economy.

"Kiev is still under threat," said one of them, who gave his name as Ruslan, a portly 43-year-old taxi driver from western Ukraine, the engine-room of Ukraine's pro-European movement.

"We have to stand here because the money for rebuilding all of this will be coming out of our pockets, via this bank," he said, clutching a wooden baseball bat.

Down the street, helmeted men in second-hand camouflage gear linked arms in a human barricade to stop curious passers-by from entering the presidential administration building.

The groups operate with the blessing of a parliament now controlled by Yanukovych's foes and a police force that melted away on Saturday when he was toppled.

On Sunday, the Interior Ministry said Kiev's traffic police would work alongside the activists to maintain order on the roads.

The exclusively male guards, many of whom are teenagers, are based in tent camps on Independence Square, where they gather around braziers or perch on barricades of tyres and torn up paving stones.

The camaraderie is infectious, but the improvised security apparatus reflects deep uncertainty over the future of Ukraine, where a fugitive president threatens to fan the flames of separatism and the economy risks collapse.

The sotnyi say they are needed to protect Ukraine's people and institutions of power during a time of political flux, at least until a new president is elected on May 25.

But it now appears their ambitions extend far beyond Kiev, into Yanukovych's eastern heartland, a collection of mainly Russian-speaking regions that are uneasy with events in the capital.

Regional leaders in the east gathered on Saturday and issued a challenge to the legitimacy of the national parliament.

The tensions are fuelling fears of Ukraine coming apart along an historic linguistic and cultural faultine between east and west.

With this in mind, Ruslan said sotnyi members were heading east.

"They're going to Donetsk, Kharkiv, Lugansk - regions where there's lots still to be done," he said.

Another, guarding the entrance to the farming ministry now occupied by men of the radical anti-Yanukovich group Spilna Sprava, also said Kiev's self-styled protectors were maintaining order in the east.

If true, the results could be explosive.

"We're seeing hundreds of volunteers every day. We're now sending some out east, to Kharkiv and elsewhere," said Lyubomir, a 58-year-old veteran of the Soviet army who said he served in the Caucasus.

Young recruits in assorted military garb traipsed past him, flashing their improvised security passes.

Some on the Maidan, however, fear the expansion east may be seen as an act of aggression, resented by easterners who look to old connections with Russia rather than Europe as a guarantee of stability.

"The police have said they support the people - it's their job to protect society now," said 27-year-old financial adviser Andrei Kondor.

"Sending these guys east could be seen as provocation. People there won't like it."

The unofficial uniform of the guards is distinctly military in style, comprised of jackboots, balaclavas, assorted camouflage, body-armour, and large metal shields.

Many of the volunteers are unemployed and find a sense of purpose among their comrades-in-arms.

They receive no pay, but if history is any guide, many may seek a place in the new political order once it is firmly in place. Ruslan, at least, said he sought no reward.

"We don't have horns, we're not devils - we're peaceful people who want their children as well as ours to live under a normal government," he said.

"When all this is over, we'll just go home and work in our gardens, but we'll always be ready at the first call."