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France has deployed hundreds of police officers to transit hubs across the country. Above, French policemen stand guard at Nice International Airport of the French Riviera city of Nice, March 22, 2016. VALERY HACHE/AFP/Getty Images

Airports and train stations across Europe beefed up security in response to a series of deadly attacks in Brussels on Tuesday. Explosions at the Belgian capital’s international airport and a downtown subway station have left at least 34 dead and about 170 injured.

Brussels' Zaventem airport remains closed after the blasts, which tore through the departures area.

France’s two busiest airports — Paris-Charles de Gaulle and Paris-Orly — are tightening up security. Earlier Tuesday, French Interior Minister Bernard Cazeneuve announced the deployment of 1,600 additional police officers across the country, including 400 at key transport hubs in the Parisian region. All flights to Belgium have been canceled.

Meanwhile, Dick Schoof, Holland’s national counterterrorism coordinator, announced extra patrols at airports in Amsterdam, Rotterdam and Eindhoven, according to Dutch media.

German federal police said it had “intensified its protective measures on the borders” and “around critical infrastructure such as train stations and air transport.” At Frankfurt’s airport, extra patrols were ordered, and officers were placed on high alert.

London Mayor Boris Johnson announced increased security at transport hubs across the city, including at Gatwick and Heathrow, Europe’s busiest airport.

Across the Atlantic, the New York City Police Department also took measures to bolster security. In a statement, the NYPD said it was deploying “additional counterterrorism resources” to “crowded areas and transit locations around the city out of an abundance of caution to provide police presence and public reassurance as we closely follow the developing situation overseas.”

“At this time, there is no known indication that the attack has any nexus to New York City,” the NYPD added.

The attacks in Brussels also saw major transit systems close down. Eurostar suspended all service to and from the Belgian capital. The Thalys rail system, which connects Paris, Brussels, Amsterdam and Cologne, said “all traffic” was “stopped.”