Cologne authorities say it's impossible to guarantee 100 percent security for the German city's famous Carnival celebrations starting this week, but security forces will be out in force to try to ensure people can enjoy the events without fear.

In the wake of a string of robberies and sexual assaults in the city on New Year's Eve blamed largely on foreigners, police chief Juergen Matheis said Monday he's doubling the number of officers on hand compared with last year to more than 2,000, and also bringing in 350 cadets.

Video surveillance is also being added, and local police are coordinating closely with federal counterparts, fire and rescue units and others.

Matheis says there's an "abstract and high" terror threat to all large gatherings that also is being taken into account.