By Marton Dunai

BUDAPEST (Reuters) - Hungary's right-wing populist prime minister Viktor Orban called on European nationalist leaders on Wednesday to revolt against an "unholy alliance of Brussels bureaucrats, the liberal world media and insatiable international capitalists".

The European Union is facing a period of upheaval following Britain's vote to leave the 28-member bloc, with nationalist parties poised to gain support in Dutch and French elections.

Orban, whose Fidesz party faces elections in 2018 after two consecutive terms in office, called on nationalist leaders to bring about a "deep but peaceful" transformation of Europe.

Right-wing leader Geert Wilders' PVV (Party for Freedom), which is anti-Islam and anti-EU, could win the largest number of votes in the Dutch polls on Wednesday, but is widely seen as being unable to form a coalition government.

"We still have a chance to keep the rising energies of revolt within a constitutional framework and transform the European empire ... but in a peaceful and orderly fashion," Orban told thousands of supporters.

Under Orban, Budapest has locked horns with European and international partners over economic and democratic issues, most recently over migration, which he says is a threat to Europe's socioeconomic make up.

Political upheaval around the world is growing because of challenges to national sovereignty from global capital and international bureaucracies, Orban said, adding that people were expressing their unhappiness with a perceived weakness in the face of mass immigration.

"To preserve our independence we need to bravely fight the battles that lie ahead of us. We must stop Brussels, defend our borders, deny mass relocations," Orban said, adding that government must be "in the hands of nationalist forces".

Opposition activists who staged a demonstration during Orban's speech used whistles to try to drown him out.

"The Hungarian people have whistled him off court," opposition politician Peter Juhasz said.

(Editing by Alexander Smith)