It was the first claim of harassment by a group of Shiite worshippers against the territory's mainstream rulers, who are Sunni Muslims. Hamas officials, who have close ties with Shiite Iran, denied the allegations.
A man who described himself as a Shiite said police burst into a house where followers were marking Arbaeen, commemorating the end of 40 days of mourning following the anniversary of the death of Imam Hussein, the grandson of the Prophet Muhammad. The man said about 15 worshippers were beaten and detained.
He declined to be identified, fearing further harassment. But some of the men filed complaints to the Gaza-based Palestinian Center for Human Rights and the rights group Mezan, which both sharply condemned Hamas over the attack.
Mezan said in a statement that during Saturday evening's incident in the town of Beit Lahia, police smashed up the apartment, broke the bones of seven of the men, detained some of them at a police station and beat them again before sending them to a military hospital for treatment.
"The attack is a violation of the freedom ... to practice one's faith," said Mezan official Samir Zakout.
Interior Ministry spokesman Ihab Ghussein said Tuesday that police stormed the apartment of a group of "outlaws" who were planning "criminal acts." He said he was unaware of the presence of any Shiites in Gaza. He said his offices would look into right groups' allegations that the men were beaten.
There are no official statistics on the number of Gaza's Shiites. They are believed to number several dozen — a minuscule minority among a population of 1.6 million people who are overwhelmingly Sunni Muslim, including the territory's Islamist militant Hamas rulers.
In many parts of the Middle East, Shiites and Sunnis have had strained relations due to deep theological differences that date back centuries. These differences have boiled over into violence over the years in places like Iraq and Pakistan.
Despite such strains, the attack in Gaza seemed surprising, given Hamas' traditionally warm ties with Iran. The fundamentalist Shiite government in Iran has given hundreds of millions of dollars, as well as military training and other backing, to Hamas.
In a separate incident, Mezan said Tuesday that a prominent rights activist who has criticized Palestinian leaders and militants was stabbed and slightly wounded by unknown assailants.
The attack on Mahmoud Abu Rahma last Friday came after he published an essay that criticized Palestinian leaders and militant groups for threatening, silencing and even harming critics.
"It is safe to assume that neither the government nor the resistance is willing to step in to protect people who dare to criticize them," he wrote.
Zakout said soon after the essay was published, Abu Rahma received threats by e-mail and text message. Hamas police condemned the attack and said they were searching for the assailants.
Additional reporting by Diaa Hadid in Jerusalem. Follow Diaa Hadid on www.twitter.com/diaahadid
On the Net: Abu Rahma's essay can be seen at www.maannews.net/eng
Shia Group 'Attacked by Police' in Gaza