Anti-government protesters were on Friday occupying regional administration buildings in six regions in western Ukraine after storming the premises, in a major new challenge for President Viktor Yanukovych.
The actions of protesters have raised the prospect of Yanukovych losing control of a vast swathe of the country as he faces mass protests in the capital Kiev.
Protesters had on Thursday seized control of the regional administration in the western city of Lviv and forced its governor Oleg Salo to write a statement of resignation.
Salo, who like all Ukrainian governors was appointed by Yanukovych rather than elected locally, later said his resignation was invalid and made under pressure.
But 200 people still spent the night in the building and showed no sign of leaving, an AFP correspondent said.
Copying the actions of protesters in Kiev, they built barricades outside the administration building made out of snow-filled sandbags and tyres.
Salo came to the building on Friday morning but, seeing the extent of the situation, left without making any comment.
Outside the city, protesters also blocked the highway leading to the Polish border.
Yanukovych has always been hugely unpopular in western Ukraine, which overwhelmingly favours integration with the European Union and a slackening of ties with Russia.
Ukraine has long been split between the nationalist and Ukrainian-speaking west and far more Russophile east which is the native region of Yanukovych.
A large number of the protesters currently massing against Yanukovych in Kiev are not from the capital but have travelled in huge numbers from western Ukraine.
There were also serious clashes overnight in the city of Cherkasy south of Kiev as protesters stormed the regional governor's offices.
However police then moved in to push out the protesters and regained control of the building, arresting 58 people and reportedly using truncheons. Activists said one young protester was badly hurt and hospitalised in a critical condition.
In the western region of Ivano-Frankivsk, thousands of protesters stormed the regional administration and managed to occupy two floors of the building despite clashes with police who used tear gas. The activists managed to occupy the offices of the governor but he was not there.
In the western Rivne region neighbouring Lviv protesters also spent the night in the regional adminstration headquarters but were allowing officials to come to work.
In the western Ternopil region protesters spent the night in the regional administration building and were awaiting for the arrival of the governor.
The regional administration has also been seized in the western Khmelnytsky region, while in Lutsk, the main city in Volyn region, police allowed protesters to enter the administration building without resistance after the governor announced his resignation.
Meanwhile, thousands of protesters Friday stormed the offices of governor Mykhailo Papiev in the city of Chernivtsi close to the border with Romania, forcing him to cede control of the building.
Channel Five television broadcast dramatic footage of protesters shouting "shame" as they break open the doors of the regional administration. Police eventually left the premises, and one official was hospitalised.
However no such actions were reported in the east of Ukraine, which has largely remained loyal to Yanukovych throughout the crisis.
Fears Ukraine could split
Ukraine's complex linguistic and cultural make-up has long created problems and some analysts have even warned that the country could one day split into two.
Most people in the west are Ukrainian-speaking and regard the Soviet rule of Ukraine as an occupation.
Around 1,000 protesters also protested outside the regional adminstration in Uzhgorod, the main city in the region of Transcarpathia on the Hungarian and Slovak borders.
They were held back by anti-riot police. Transcarpathia is generally seen apart from the rest of western Ukraine due to its complex ethnic and linguistic mix.