The Flemish far-right party Vlaams Belang (Flemish Interest) could take control of the northern port city of Antwerp after partial results from state television suggested it will make a strong showing in the city.
The party has been kept in opposition by a coalition whose only common cause has been keeping Antwerp out of the hands of the anti-immigrant party.
Vlaams Belang won 33 per cent of the vote in the 2000 municipal polls in its former guise as Vlaams Blok, which was found guilty of racism in 2004.
Filip Dewinter, the party's leader, was optimistic that his party could make another breakthrough after successes in local elections six years ago and regional elections two years ago.
"In a normal democracy, a party that sees its support increase from zero to more than 33 per cent over 24 years should be part of a governing coalition," Dewinter said.
The last opinion poll showed it had 25 per cent support in the Flemish part of the country where it hopes to unseat the Christian Democrats as the biggest single party.
Filip Dewinter has focused on
crime and immigration
A reinforcement of the extreme right in Flanders would complicate the situation for Guy Verhofstadt, the prime minister, whose party is struggling.
Verhofstadt's Dutch-speaking Liberal Democrats have taken major losses across many towns across Flanders in northern Belgium, according to early results and exit polls.
But a racially motivated killing in May could work against Vlaams Belang. Thousands of Belgians have taken part in anti-racism marches since the murder in which a skinhead shot an African woman and a white child in her care.
Weak on crime
Dewinter's rallying cry has been "Belgie Barst", or "Belgium explode".
The party, and others on the Flemish right, have accused French speakers of being weak on crime and immigration, with the issues widening divisions between the 6.5 million Flemish and 4.5 million French-speaking Belgians
Francophile parties in the south could also be hit by gains by the French-speaking far right.
The dominant Socialist Party is expected to be badly hurt by a financial scandal over state housing and in Charleroi the extreme-right National Front is expected to get between 15 and 19 per cent of the vote.
About 7.5 million registered voters were asked to vote in 589 municipalities - 308 in the Flemish-speaking north, 262 in Francophone Wallonia and 19 in Brussels.