After the Roman Empire was divided, Thessaloniki was the second most important city in the Byzantine Empire (after Constantinople).

Several of the churches in the city are very old, but they were badly damaged in various earthquakes and a huge fire in 1917. So they have generally been reconstructed in the last century.

St. Sophia's, which dates from the 7th or 8th century (depending on which guidebook you believe).

There were little shrines on each side of the gate, containing candles and icons. I saw this outside the gates of other churches too.

I didn't take any pictures inside St. Sophia's, because everyone inside was acting like parishioners rather than like tourists and I didn't want to be disrespectful. It was very dark in there -- not many windows. (In fact, I seem to have completely missed a famous mosaic.) St. Dimitrios's was a lot brighter.

This is St. Dimitrios's. It's named for the patron saint of the city, a Roman scholar and officer who converted and was martyred (put to death by Galerius, who persecuted Christians). This church is known as the oldest church in the city (after the Roman Rotonda, I guess), but it was badly damaged in the 1917 fire (and one in AD 620!), so a lot of it has been reconstructed. It's also the largest church in Greece. It spent some time being a mosque when Thessaloniki was controlled by the Ottoman Empire.

There were a number of mosaics inside, which were quite beautiful, but most are modern reconstructions.

This mosaic fragment is now in the Museum of Byzantine Culture. It was rescued from St. Dimitrios's after the 1917 fire.

Also from the museum, a pillar from an early church (4th century?)

And here's a church I just happened to walk by.