Hamas security forces beat many of the 200 Fatah prisoners they arrested last weekend in the largest crackdown by the Islamist movement on its secular rivals since they seized power in the Gaza Strip 13 months ago, human rights monitors claim.

The crackdown was launched after a bombing in a beachfront café in Gaza City last Friday which Hamas blamed on Fatah. A four-year-old girl and five Hamas militants were killed, and 27 people were injured. Fatah denied involvement.

Forces loyal to President Mahmoud Abbas in the West Bank responded to the beatings with up to 100 arrests of Hamas leaders and activists, including three imams in and around the northern city of Tulkarm.

Fred Abrahams, a researcher for the New York based Human Rights Watch group, who visited Gaza on Tuesday, said: "Of three [Fatah] people we met who were released, one had been moderately beaten and one seriously beaten. We have serious concerns that this was widespread." Given a past "pattern of abuse", there was also concern that forces loyal to Mr Abbas had mistreated Hamas prisoners, he added.

As part of the Hamas campaign, more than 40 Fatah-affiliated organisations in Gaza have been shut down. These include political offices, the Wafa news agency, three associations for the rehabilitation of the handicapped, students and women's groups, an agricultural training centre and government offices under the jurisdiction of Mr Abbas. Offices belonging to Ziyad Abu Amr, an independent MP who ran with Hamas's backing in the last elections, were also shut.

Meanwhile, Palestinian human rights group al Haq warned that both Hamas and Fatah were using arrest and torture against their political opponents. The group's director, Shawan Jabarin, estimated that more than 1,000 people had been seized by each side before the most recent unrest, out of whom up to one third had been tortured. Human Rights Watch backed the findings in their own report on abuse, released this week.