Environmental group WWF on Thursday attacked the Bulgarian parliament's decision to relax planning restrictions on developing forests, farmland and other protected areas.

"The controversial amendments (...) will lead to the plundering of the country's last significant natural resource," WWF warned in a statement, calling on President Rosen Plevneliev to veto the amendments.

Parliament adopted the measures on Wednesday.

The changes would relax rules on building ski runs and facilities in protected areas without the previously required change-of-land-use permits.

Parliament's decision to approve the changes prompted several hundred nature lovers to block the main road into downtown Sofia Wednesday evening and police arrested 12 demonstrators during the protests.

The group would also alert the European Commission, arguing that the amendments amounted to "unauthorized governmental aid", WWF Bulgaria's communications chief Konstantin Ivanov said.

Bulgarian green campaigners have also argued that the changes to the law would favour recent efforts by a ski run operator on Mount Vitosha, near Sofia, to expand existing ski runs and facilities into protected nature areas.

The WWF said any such change to the law would let private companies build on state-owned land that was meant to be protected and open the way for farmland to be used for ski resorts and golf courses.