GAZA CITY, Israeli-Occupied Gaza Strip — Written in blood-red paint, the message on the wall of the mosque was a taunting battle cry: "To Fatah, the cowards and surrenderers: Where are your bullets, and why are they not aimed at the Israeli occupiers? Or are your targets the sons of our own people?"

Signed by the Islamic Resistance Movement, the message was aimed at the Palestine Liberation Organization's mainstream Fatah group, and it appeared calculated to provoke further clashes between the two movements' supporters.

"Fatah is being unmasked," said Dr. Mahmoud Zahhar, a Gaza surgeon who supports Hamas, or Zeal, as the Islamic Resistance Movement is known from its Arabic initials. "Fatah pretends to fight, but it attacks not Israelis, but Hamas.

"Fatah says it is the vanguard of Palestinian nationalism, but actually it is in retreat. Fatah says it speaks for the people, but through this 'peace process' it actually collaborates with the Israelis in perpetuating our occupation."

The Hamas challenge is real enough. Its rivalry with Fatah has resulted in street battles, firebombings, abductions and outright assassinations. A cease-fire reached in July broke down last month, and a neighborhood-by-neighborhood, mosque-by-mosque, school-by-school struggle for domination of the area resumed.

"The Islamic resurgence is rising with the power and the inevitability of the ocean tide," Zahhar said, "and once people commit themselves to Islam they remain committed. . . . We will have an Islamic government here in time."

But Tawfiq Abu Khousah, a journalist jailed previously by Israeli authorities as a senior Fatah cadre, dismissed the Hamas challenge as "a childish diversion" from what he called the main Palestinian issue, "establishing our own state in the Gaza Strip and in the West Bank."

"Hamas wants the peace process to fail so it can replace Fatah and the PLO, and so it attempts to divide us and sap our energy with these attacks," Abu Khousah said. "But Hamas itself will fail. The masses support Fatah, and the masses recognize that the most effective strategy we have now includes the Washington talks."

On an initial level, Hamas' sustained challenge to Fatah, not only in the Gaza Strip but across much of the Israeli-occupied West Bank and in Jerusalem as well, is over Palestinian participation in the U.S.-sponsored Arab-Israeli negotiations, which Hamas opposes.