According to media reports, people broke through the barriers and attempted to approach the vehicles.
However, they were immediately stopped by security forces. The activists wanted to protest against Spanish bullfighting, in which animals are killed unnecessarily and cruelly, according to reports.
They were two people from Germany and one each from England and Spain. Similar protests had already taken place several times before, most recently in August in the Vatican Audience Hall.
On the occasion of the Solemnity of the Virgin Mary, conceived without original sin, the Pope traditionally visited the Marian column in Rome's historic centre near the Spanish Steps.
During a short ceremony, Francis brought a large arrangement of roses to the twelve-metre-high statue of the Virgin Mary. He then allowed himself to be driven in a wheelchair to the many people at the side of the road, shook hands and distributed sweets and rosaries.
The 87-year-old took a lot of time for the many people with disabilities who were waiting for him in wheelchairs.
Pope: Building sites in Rome are annoying, but necessary
In a speech, the Pope asked the Romans for understanding for the many construction sites in the city. "Rome is preparing for the Holy Year, which will be a message of hope for humanity, which is plagued by crises and wars," he said.
The construction sites throughout the city caused many inconveniences, "but they are also a sign that Rome is alive and renewing itself to become more welcoming and functional," Francis said. "The mayor is preparing everything here for the Holy Year so that it will be good," the Pope said in the presence of Rome's mayor Roberto Gualtieri. "Let us pray for the mayor, who has a lot of work to do!"
Francis will open the Holy Year on 24 December.
Around 32 million visitors are expected to attend the feast year, which normally only takes place every 25 years.
Afterwards, Francis visited the nearby Palazzo Cipolla to view the painting "White Crucifixion" by the Russian-French painter Marc Chagall (1887-1985), which is considered to be the Pope's favourite painting. It will be on display there until 27 January as part of the Holy Year 2025.
Prior to this, the Pope visited the "Salus Populi Romani" image in the Basilica of Santa Maria Maggiore on 8 December, as he does every year, and honoured it with a bouquet of flowers.
Francis also traditionally visits the icon in his favourite Roman church before or after trips.
This time he entrusted the image of the Virgin Mary with his trip to the French island of Corsica next Sunday.