Belgium's government has kept the capital on the highest state of alert in the face of what it described as a "serious and imminent" threat, but it's not the first time a city has been shut down by an emergency.

Here are a few examples:

BOSTON

Massachusetts Gov. Deval Patrick issued a call for the population to shelter-in-place on April 19, 2013 after police engaged in a gunbattle with two people suspected of setting the bombs at the Boston Marathon a few days earlier. Patrick shut down the region's public transportation system, stores were shuttered and streets deserted in a move that affected more than 1 million people in the greater Boston area. Convicted bomber Dzhokhar Tsarnaev was captured shortly after the shelter-in-place request was lifted about 12 hours later. Tsarnaev has been sentenced to death in the attack that killed three people and injured more than 260.

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MEXICO CITY

Within five days of the April 23, 2009 swine flu announcement, authorities in Mexico issued orders that essentially shut down the city, closing schools, museums and, bars, movie theaters and discos — emptying the streets of traffic. Businesses were closed for only a few days, but the alert by health authorities lasted more than a year. The pandemic killed 1,289 in Mexico.

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FREETOWN, Sierra Leone

Authorities declared a state of emergency in 2014 and once locked down the entire country, banning activities that could further the spread of the highly contagious Ebola virus. Troops were brought in to enforce quarantines and stop infected people from traveling. Schools were shut down for several months.

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NEW YORK

Precautions were taken after the Sept. 11 attacks. The Federal Aviation Administration shut down airports nationwide. In New York City, elections were called off, airports closed and trading on Wall Street suspended. Rockefeller Center property managers urged tenants to go home. In New Jersey, airport and river crossings to New York were closed and commuter trains were canceled.