(Recasts with Abbas orders to release prisoners)

NABLUS, West Bank, July 31 (Reuters) - Palestinian President Mahmoud Abbas ordered his security services on Thursday to release all pro-Hamas activists arrested in the occupied West Bank this week, the official Palestinian news agency Wafa said.

Dozens of Palestinians have been arrested in tit-for-tat sweeps by Fatah in the West Bank and Hamas in the Gaza Strip following a series of bombings last week in the coastal enclave, which Hamas seized a year ago.

Hamas blamed Fatah for the blasts, a charge Fatah denied.

Palestinian security officials said they had heard the report of the order from Abbas, but had not yet received official instructions to begin freeing the Hamas prisoners.

Abbas's decision followed Wednesday's release of more than half the 200 Fatah activists rounded up by Hamas in Gaza.

Earlier on Thursday, Palestinian security forces arrested 15 Hamas activists in the West Bank city of Nablus, including four university lecturers at Al-Najah University.

Pro-Abbas forces in the West Bank city of Jenin also arrested 17 supporters of Hizb ut-Tahrir, an Islamist group whose goal is to establish a pan-national Muslim state including the Palestinian territories and Israel.

A Palestinian security officer said the arrests were made during an "illegal demonstration" by the group, adding that the men would be questioned and possibly charged. (Additional reporting by Mohammed Assadi and Ali Sawafta in Ramallah and Nidal al-Mughrabi in Gaza and Haitham Tamimi in Hebron) (Reporting by Atef Sa'ad; writing by Joseph Nasr and Avida Landau in Jerusalem; editing by Andrew Roche)

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