(Adds Hamas spokesman, restoring control, Egyptian measures)

RAFAH, Egypt, July 2 (Reuters) - Palestinians clashed with Egyptian police at the Rafah border crossing between Egypt and the Gaza Strip on Wednesday when some of the Palestinians tried to force their way across, witnesses said.

Dozens of Palestinians from the territory, controlled by the Islamic militant group Hamas, pelted Egyptian border police with stones, injuring at least six of them, Egyptian security and medical sources said.

Live television footage showed Egyptian forces firing water cannon and hurling rocks in response as they sealed the gates to the crossing, the only corridor between Gaza and Egypt. It has mostly been closed since Hamas's takeover of Gaza in 2007.

Palestinian officials and witnesses said Hamas had beefed up security at the site after the violence and was restoring control, ordering people to leave and forcing the crowd back across the Palestinian border.

The border opened on Tuesday to let sick and injured Palestinians through, as well as Palestinians with residence or work permits in third countries, and to allow Palestinians to return to their homes from Egypt. Officials said then that they planned to keep it open for three days.

But Palestinian officials and witnesses said crowds had become frustrated by the slow pace of entry, with only around 200 Palestinians being allowed through on Tuesday out of the thousands seeking entry, according to Egyptian security sources.

Hamas spokesman Sami Abu Zuhri told Reuters in Gaza, "It (the violence) was a spontaneous act that reflected the state of pressure and suffering the Palestinian citizens are being exposed to in the Gaza Strip... The incident stresses the need to open the Rafah crossing permanently to Palestinian citizens."

An Egyptian security source said Egyptian authorities were threatening a total closure. A Palestinian official in the Gaza Strip said Egypt had closed the border for the day, but would process any Palestinians inside the terminal.

The security source said Egypt was moving truckloads of riot police to Rafah to reinforce the police presence there, and that authorities had received instructions to stop Palestinians in Egypt seeking to return to the Gaza Strip from reaching the border area.

Hundreds of thousands of Gazans, subject to an Israeli-led blockade exacerbated by Egypt's border closing, crossed into Egypt in January to stock up on supplies after Hamas militants blew breaches in the border wall. (Reporting by Yusri Mohamed; Additional reporting by Nidal al-Mughrabi in Gaza; Writing by Aziz El-Kaissouni; Editing by Caroline Drees)

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