BRUSSELS (Reuters) - Belgium's king has angered separatists and some constitutional experts by making what they saw as a veiled criticism of the main political party in the Dutch-speaking north.

King Albert II used his Christmas message to warn against "populist" political sentiment that rises during economic crises, drawing parallels with the pre-Nazi era in Europe when foreigners were made scapegoats for the problems.

"Such argument is common in many European countries today, also in ours. The crisis of the 1930s and the populist reactions that it caused must not be forgotten," the king said in a speech broadcast on Christmas Eve.

Carl Devos, politics professor at Ghent University, said the king - supposed to be a unifying force in a country sharply divided between the French-speaking south and the Flemish north - should not have made the comment which he said was a tacit reference to the separatist N-VA party.

"It's weird for the head of state to refer to the largest party of the country with a reference to the 1930s," Devos told Reuters.

The N-VA, which wants Dutch-speaking Flanders to split from Belgium, won the most seats in the 2010 federal election.

N-VA's leader, Bart De Wever, who is mayor of Flanders' biggest city, Antwerp, accused the king of being "divisive" and of favoring the French-speaking Socialists of Prime Minister Elio Di Rupo.

"His Christmas message was an unhappy pinnacle," he wrote in an editorial De Standaard newspaper on Thursday.

De Wever said the king should draw his own lessons from the 1930s and the example of Albert's father Leopold III who surrendered to Germany in 1940, raising questions about his loyalty.

Although constitutionally above day-to-day politics, Albert was involved in appointing mediators in negotiations to form a government after the 2010 election, a process that took a world record of almost 18 months.

De Wever argued that the king should no longer be involved in the process after the next federal election, due in 2014.

Dave Sinardet, political scientist at the VU Brussels university, said there were questions over who had written the speech, which in theory would have been drafted by the king himself before being vetted by the government.

"The question is who held the pen when writing the passage about the 1930s," Sinardet told Reuters. "Were there corrections? Was it added later? That's not clear at all."

(Reporting By Philip Blenkinsop and Robert-Jan Bartunek; Editing by Robin Pomeroy)