British police on Monday launched a new advertising campaign which urges citizens to watch out for unusual activities _ and to call detectives if they suspect behavior that's "not quite right." The counterterrorism campaign is aimed at anyone who visits London, as well as residents and commuters, the Metropolitan Police said in a release. It is to last five weeks. "We want people to look out for the unusual _ some activity or behavior that strikes them as not quite right," said Deputy Assistant Commissioner Peter Clarke, head of head of London's anti-terrorist police. "We are asking members of the public to trust their instincts." The hotline is not new, but the print and broadcast campaign urges people to use it. One of the radio ads features a woman asking, "How do you tell the difference between someone just videoing a crowded place, and someone who's checking it out for a terrorist attack?" A man replies, "You don't have to," and urges a call to the number, where specialist officers will evaluate the information. Print ads warn of people who visit "terrorist" web sites, who buy large amounts of chemicals for no obvious reason and who have documents in different names.