"They barged in spraying gas in the faces of people from those spray cans, and went about dishing out blows and shouting, about 15-20 patrol cars turned up at the church, outside which some 600 people had gathered, many of them from a protest march that had just ended.
Some 25 dissidents dressed in black had walked inside the church to protest the arrest of another government opponent, said Elizardo Sanchez, president of the Cuban Human Rights and National Reconciliation Commission.
"The repressors, headed by a lieutenant colonel and other state security officers, desecrated the church of Santa Teresita after kicking one of its doors open and savagely assaulting the peaceful dissidents," he said in a statement.
Sanchez, whose organization is outlawed, later said that eight detainees had been let go by authorities, but that "seven remain under arrest."
He said the crackdown was an "extremely serious act of political repression with practically no precedent." The commission said it "hopes the government will conduct a serious investigation and stop encouraging or allowing premeditated and unnecessary acts of police brutality against citizens trying to exercise their right to demonstrate."
Sanchez said the police action was part of "a policy of preventive repression" ahead of Human Rights Day on December 10 when several opposition members have scheduled events.
Santiago de Cuba Archbishop Dionisio Garcia also voiced concern.
"We're not used to this. I had no idea uniformed police could do that ... we're talking now to avoid such incidents in future," he said, adding he would meet with government officials on Thursday.
Conrado Rodriguez said that as the dissidents were rounded up, he told the police: "I want you to explain to me what is going on here, because I don't understand anything. How is this act of violence possible?"
Sanchez's group says there are about 250 political prisoners in Cuba.