Like any international metropolis, Prague attracts tourists, but also petty crime. You don't need to worry that you will automatically be robbed in Prague, but it's better to be take precautions in advance.
Pickpockets most commonly operate in the historical centre, especially on Královská cesta (Royal Way) from Prague Castle to Old Town Square, the vicinity of Národní třída, near the main railway station and Florenc bus station. Pickpockets often target crowds of tourists, selecting individual victims to rob. They also make use of crowded trams and metro trains. You should therefore take care of your belongings on public transport in the city centre.
If you become the victim of a crime, call the police on 158.
How to avoid being robbed in the street:
Always keep your belongings in a safe place, in an inside pocket of clothing or in closed baggage.
When going on trips, don't carry a lot of cash, leave important documents and jewellery in your hotel safe.
Don't leave a purse/wallet or personal baggage unattended in the street, in shops or restaurants. Don't put mobile phones down anywhere.
Don't take out your purse/wallet unnecessarily: refuse street offers to change money, provide drugs or any other goods.