The tests are provided at a clinic under the colonnades of St Peter’s Square - established at the request of Pope Francis several years ago - which offers the homeless showers and haircuts as well as medical services.
Archbishop Rino Fisichella told that some 50 coronavirus tests a day were being done and the initiative would continue indefinitely.
The pope established the World Day of the Poor four years ago to bring attention to the needy, particularly in developed countries. It is being marked this Sunday.
Because of coronavirus restrictions, only 100 poor people will attend Mass in the vast basilica.
The lunch has been cancelled but about 5,000 food parcels will be delivered to poor families, including 2.5 tons of pasta donated by an Italian food company.
More than 350,000 masks will be given to students in Rome’s less affluent outskirts.
As we reported earlier, the Vatican has sought to clarify Pope Francis' comments in support of same-sex civil unions, saying they were taken out of context and do not signal a change in doctrine.