Police in the Netherlands are appealing for calm as they investigate what they believe is the latest in a spate of arson attacks against religious targets.A mosque at Helden near the German border in the south of the country has been gutted. There have now been at least 20 arson attacks on mosques and churches since the murder of filmmaker Theo van Gogh by an Islamic militant 11 days ago. “People were keeping an eye on the mosque,” said a Helden police official. “It wasn’t permanently guarded though. After the murder of van Gogh we did talk about increasing security here, but we hadn’t actually finalised the plans.” In a country known for its tolerance, police say they have arrested several dozen people in connection with arson attacks, bomb threats, letters containing suspicious powder and far-right vandalism. The murdered filmmaker had angered some Muslims with a movie that accused Islam of promoting violence against women. In the Hague on Wednesday, two men were arrested after a 14-hour standoff in which suspects threw a grenade, wounding four officers. The pair, along with four others, now face charges of belonging to a terrorist organisation. A seventh man is accused of involvement in the Van Gogh murder. Some reports say the two men arrested in the Hague are suspected of plotting to assassinate some outspoken Dutch politicians who have criticised Islam. The Netherlands Queen Beatrix met with Moroccan and Dutch students in Amsterdam on Friday, and stressed the importance of respect and tolerance of all groups living in the country.