The activists were affiliated with the Ultima Generazione (Last Generation) group, which aims to raise awareness on climate crisis and urge the government to stop the use of carbon fuels, ANSA news agency reported.
Police said the liquid was vegetable liquid coal, and the activists were arrested and could be charged for damaging cultural heritage.
Mayor of Rome Roberto Gualtieri condemned the action and went to the famed site after the incident.
“We share the issues of raising awareness of citizens on the risk of climate change, but this is not the right method. It is not by risking damaging our heritage that we help the environment,” he said.
Culture Ministry Gennaro Sangiuliano, for his part, said “anyone who damages our cultural heritage cannot get away with it and must be severely punished.”
He added that an investigation is now underway to assess the extent of the damage.