This memorial is in Piazza Trilussa, along the Tiber River in Trastevere. Trilussa was the pen name of Carlo Alberto Salustri (1871-1950) who is known for poems written in the Roman dialect. ("Trilussa" is an anagram of “Salustri”.)
This is a view of the piazza with the fountain built by the Borghese Pope Paul V.
Senatus Populusque Romanus
"The Senate and the People of Rome"
It originally referred to the government of the ancient Roman Republic, and used as an official signature, appearing on coins, civic inscriptions, and on the standards of the Roman legions. Today, SPQR is the motto of the city of Rome and appears in the city's coat of arms, the city's civic buildings, manhole covers, billboards and even on memorial wreaths!