"We do not know his name or his whereabouts, only that it was somewhat violent," said Elizardo Sanchez, head of a group that monitors detentions of government opponents.
He urged Cuban authorities, who have not commented on the incident, to identify the man.
Benedict seemed to walk with renewed vigor Tuesday as he greeted officials and clergy when his plane arrived in Havana. The previous evening, his spokesman acknowledged that the pope was fatigued from days of traveling in Mexico.
He was greeted on the tarmac by clergy, government officials and children who played music, danced and offered him flowers.
Ana Blanco, a 47-year-old Havana resident complained about people being told to attend a papal Mass on Wednesday in Havana, saying the pressure seemed odd in a country that in her early years taught her religion was wrong.
"Now there's this visit by the pope, and I don't agree with giving it so much importance or making anyone go to the Mass or other activities," the office worker said. "Before it was bad, now it's good. That creates confusion."
Miami Archbishop Thomas Wenski, who led a pilgrimage of about 300 mostly Cuban-Americans to the island for Benedict's visit, got a sustained standing ovation Tuesday when he gave a homily in a Havana cathedral packed mostly with Floridians. Wenski called for increased respect for human rights and political change on the island, while also warning against unbridled capitalism.
"The Pope and the Cuban Church want a transition that is dignified for the human being, dignified for Cubans," he said in Spanish. "The Church wants a soft landing ... and a future of hope."
Venezuelan president Hugo Chavez, in Havana receiving radiation treatment for cancer, sent his greetings to Benedict, but said there was no plan to meet with the pope: "They have their agenda. I'm not going to be interfering at all."