A student runs for cover as riot police clash with students during a demonstration on February 29, 2012 in Barcelona, Spain. Student associations have called upon 120,000 students to strike in protest against education cuts recently announced by the Catalan government. (Photo by David Ramos/Getty Images)

Chinese special policemen rank in a line of battle during an anti-terror drill at a training base of Chengdu Armed Police Headquarters on September 28, 2005 in Chengdu of Sichuan Province, China. (Photo by China Photos/Getty Images)

A simulated casualty is evacuated during an emergency services exercise at the disused Aldwych underground station on February 22, 2012 in London, England. Operation Forward Defensive aims to test the response to a terrorist incident on the London Underground network during the 2012 Olympic Games. (Photo by Peter Macdiarmid/Getty Images)

A security exercise takes place along the River Thames on January 19, 2012 in London, England. The exercise including around 44 police officers, 94 military personnel, 15 boats and a Royal Navy Lynx helicopter was conducted by both the Metropolitan Police and the Royal Marines and designed to test their joint capability ahead of the 2012 London Olympic Games. (Photo by Dan Kitwood/Getty Images)

Interpolitex - the largest national forum of homeland security is the only national security fair of federal status and importance. It is held to provide manufacturers, suppliers and users special equipment and weaponry with an efficient tool for implementing a wide range of advertising and marketing objectives, monitoring of the market and deal-making. (Russia, Moscow, October 2011)

A UCLA student is arrested by Los Angeles Police Department officers after she attempted to escape after eleven student protesters sat in circle in middle of the Westwood and Wilshire boulevard intersection as part of a protest of bank practices and rising fees at public universities on November 9, 2011 in Los Angeles, California. The protest organized by ReFund California was one of several planned at universities around the state. (Photo by Kevork Djansezian/Getty Images)

Metropolitan Police officers prepare to carry out a raid on a property on the Churchill Gardens estate in Pimlico during Operation Woodstock on August 11, 2011 in London, England. Over 1,000 people have been arrested since the rioting began on Saturday and Police have started to raid properties across the capital as they round up people suspected of involvement in the rioting and recover stolen property. (Photo by Anthony Devlin - WPA Pool/Getty Images)

A British policewoman chasing after a group of naked street boys by the Serpentine in Hyde Park, London. (Photo by Reg Speller/Getty Images). 1926