Some of the world’s top chess players have decided to boycott this year’s Women’s World Championship in Iran because they refuse to wear headscarves as required by that country’s Islamic government.

The international tournament starts in Tehran on Feb. 10, but nine of the 64 players who qualified are refusing to go. Four of them have said publicly that the boycott is in protest of Iran’s restrictive laws against women.

One of the most outspoken chess players is Argentina’s Carolina Lujan, who withdrew from the championship after the World Chess Federation told her she would have to abide by Iranian headscarf laws during the tournament. Lujan was also told she would not be allowed to be alone with a man in private—a restriction she says could hinder her ability to meet with her coach before each match.

“The obligatory use of the hijab is not just a simple dress code,” Lujan wrote on Facebook. “It means a lot to me, and due to my beliefs and values I am not going to be forced into using it.”

Lujan added in a follow-up message, “In my country…we do not force Muslims to remove their hijab. We respect their choice.”