ANKARA, May 6 (Reuters) - Turkey's main opposition said on Monday it was a "plain dictatorship" that had decided on a re-run of local elections in Istanbul, after the election board decided to annul vote results showing a painful defeat for President Tayyip Erdogan's party.

"It is illegal to win against the AK Party," main opposition CHP party Deputy Chairman Onursal Adiguzel said on Twitter. "This system that overrules the will of the people and disregards the law is neither democratic, nor legitimate. This is plain dictatorship." (Reporting by Ece Toksabay Editing by Peter Graff)