The American Embassy in Latvia has posted the following warning:
Please be aware that there are numerous scam artists targeting foreigners in the tourist pubs and restaurants. There have been a number of reports of foreign tourists being charged exorbitant prices for drinks in bars; some have then been assaulted or forced to withdraw money from an ATM to pay the bill. You can avoid situations like this by ensuring that you check the price of drinks before ordering, pay for one round at a time, and seek recommendations for bars from trustworthy sources such as Riga In Your Pocket. If possible, you should avoid walking alone at night and avoid using ATMs after dark.
The Riga Tourist Police Unit has a 24/7 English-speaking operator at (+371) 6718 1818 and English-speaking officers who frequently patrol the Old City. The U.S. Embassy can be reached 24/7 at (+371) 6710 7000.