WASHINGTON (Reuters) - American Muslims have launched an advertising campaign to denounce acts of terrorism after bombers believed to be British Muslims killed at least 54 people in attacks on London. "Any effort by terrorists to hide their criminal activities under the mask of religious piety is being categorically and unequivocally rejected by mainstream Muslims," said Parvez Ahmed, chairman of the Council on American-Islamic Relations. He said the television ad, which will air nationwide by July 19, is an attempt to detach Islam from the "heinous" acts of a few Muslims. Police believe the attacks are linked to al Qaeda,...