Lisbon is pretty much a safe city, with a very low crime rate and rare violent crime. Like most european capitals, some precautions should be taken, specially regarding pickpockets and belongings left unattended in restaurants, coffee shops, cars, etc. During day, most areas in Lisbon are safe, but at dawn, after sunset, there are some areas in Lisbon that should be avoided, specially for tourists.
One of the areas is around Martim Moniz and Intendente – map below (number 1 and 2)
It’s close to the center and has big communities of emmigrants. Intendente should be avoided on either way, but Martim Moniz if you are walking with a group should be fine.
The area of Cais do Sodre (map below – number 3) is also safe during day, and quite busy with bars after dinner and up to 1AM. After that time, it’s not recommended for tourists to walk around alone around that area. There’s people getting the train and the boat from this point, and often teen groups/gangs that can cause some troubles.