San Giorgio al Velabro in the Velabrum-Forum Boarium, Rome, interior columns taken from sundry Roman temples.

San Giorgio in Velabro is a church in Rome, Italy, devoted to St. George. The church is located in the ancient Roman Velabrum, near the Arch of Janus, in the rione of Ripa. Sited near the River Tiber, it is within a complex of Republican-era pagan temples associated with the port of Rome. The ancient Arcus Argentariorum is attached to the side of the church's façade.
The current church was built during the 7th century, possibly by Pope Leo II, who dedicated it to Saint Sebastian. Its plan is irregular, indeed slightly trapezoidal, as a result of the frequent additions to the building. The interior columns are almost randomly arranged having been taken from sundry Roman temples.
Between 1923 and 1926, the Superintendent of Monuments of Rome, Antonio Muñoz, completed a more radical restoration programme, with the aim of restoring the building's "medieval character" and freeing it from later additions. This was done by returning the floor to its original level (and so exposing the column bases) reopening the ancient windows that gave light to the central nave, restoring the apsis, and generally removing numerous accretions from the other most recent restorations. During this process, fragments (now displayed on the internal walls) were found indicating a schola cantorum on the site, attributed to the period of Gregory IV (827–844 AD.). The building as we see it today is largely a product of the 1920s restoration.

Fragments of the dedicatory inscription from the nearby Arch of Janus are still preserved inside church of San Giorgio al Velabro.

More on https://en.wikipedia.org/wiki/San_Giorgio_in_Velabro , https://en.wikipedia.org/wiki/Arch_of_Janus and http://www.roma-antiqua.de/antikes_rom/circus_maximus/forum_boarium