By Alessandra Prentice

KIEV, Feb 24 (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 Yanukovich.

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 Yanukovich 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.

POLITICAL FLUX

The groups operate with the blessing of a parliament now controlled by Yanukovich'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.