Several thousand Bulgarians demonstrated across the country on Saturday against plans for shale gas exploration by US company Chevron that they say could harm the environment.

About 1,000 youngsters marched along the streets of the capital Sofia, beating drums and blowing whistles as "a wake-up call to all Bulgarians," an AFP correspondent at the scene reported.

They urged people to push the government to impose a ban on hydraulic fracturing or "fracking", the most commonly used method for shale gas exploration that opponents say might contaminate drinking water.

The protestors marched on the government buildings carrying banners saying "No to shale gas, Yes to nature", and "Chevron go home" to protest the US company's plans to extract shale gas in the European Union's poorest member.

Similar-sized demos were held in the northeastern region of Dobrudzha, where Chevron wants to operate, as well as in the Black Sea cities of Varna and Burgas, Plovdiv in the south and Pleven to the north, national radio reported.

Environmental pressure group Fracking Free Bulgaria said Saturday it had obtained a promise from parliamentary speaker Tsetska Tsacheva for lawmakers to consider a moratorium on fracking for shale gas.

Similar legislation was already passed in France last June.

Fracking, which uses high pressure injections of water, sand and chemicals to blast through rock to release oil and gas trapped inside, has been widely used in the United States.

Bulgaria, which depends almost totally on Russian natural gas deliveries via Ukraine, has seen its shale gas potential as a possible way to diversify supplies.

Bulgaria's government granted Chevron last June a five-year testing permit to explore a potentially huge field near Novi Pazar in northeastern Bulgaria.

The country's reserves are estimated at between 300 billion and one trillion cubic metres of shale gas, Energy Minister Traicho Traikov had said, citing data by companies.

Environmental groups claim that some preliminary drilling was already done in the northeast as well as in the Black Sea shelf but the government has not confirmed this.