(adds details of planned return to Cyprus)

SDEROT, Israel, Aug 26 (Reuters) - Police on Tuesday arrested an Israeli peace activist who had sailed to the Hamas-ruled Gaza Strip to challenge Israel's blockade of the coastal enclave.

They accused Jeff Halper, who also has U.S. citizenship, of violating a ban on Israelis entering Gaza.

Halper was among 44 "Free Gaza" activists from 17 countries who sailed in two boats from Cyprus to the Gaza Strip on Saturday in defiance of the blockade.

The activists plan to sail back to Cyprus on Thursday and have vowed to take several Palestinians with them, including students prevented by Israel from leaving Gaza to study abroad.

An Israeli Foreign Ministry spokesman said Israel had not yet decided whether to prevent the Palestinians from sailing from the Gaza Strip with the activists.

Halper spent three days in the territory before entering Israel through the Erez border crossing, where he was arrested.

"He is being questioned at the police station in Sderot for entering the Gaza Strip in defiance of a military decree banning Israeli citizens from doing so," Israeli police spokesman Micky Rosenfeld said.

Israel allowed the activists to sail to the Gaza Strip, the first foreigners to reach the territory by sea since travel restrictions were tightened after Hamas's takeover more than a year ago, saying it wanted to avoid a public confrontation.

As part of an Egyptian-brokered ceasefire that took effect in June, Israel has eased its blockade of the territory, allowing in more humanitarian goods and medical equipment. (Reporting by Yehuda Peretz and Joseph Nasr; Writing by Joseph Nasr; Editing by Andrew Dobbie)

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