The U.S.-Mexico border is one of the busiest in the world with millions of people and goods crossing each day.

CNN: Commentary: Flu shouldn't close Mexico border

The department, which is one of the busiest in the East Midlands, treats more than 400 people a day and more than 150, 000 a year.

BBC: Queen's Medical Centre A&E reports 7% rise in patients

Our reporter says the courts are located in one of the busiest parts of the city, and were full of people when the attacks happened - Sunday is a normal working day in Somalia.

BBC: Gunmen launch deadly attack on Somalia courthouse

The Camp Bastion complex, near the large U.S. Camp Leatherneck, is one of the busiest airfields in the world, with more than 28, 000 people working on site, according to the UK Ministry of Defense.

CNN: UK's Prince Harry deployed to Afghanistan

At 6:30 p.m. a train from Shinjuku station, one of the busiest in Tokyo, was packed with people rushing home before a possible blackout.

FORBES: Tokyo Streets Quiet As Foreigners Flee, Locals Hunker Down

The force said hundreds of people may have witnessed the stabbings, which happened during rush hour in one of the city's busiest areas.

BBC: Birmingham stabbings: Attacker 'knew Big Issue victims'