CAIRO, May 18 (Reuters) - Egyptian police angered by the kidnapping of seven colleagues by Islamist gunmen kept a crossing into the Gaza Strip closed again on Saturday, stranding hundreds of Palestinian travellers, witnesses said.

The protest began on Friday when police strung barbed wire across the Rafah border post and chained up the gates, local residents said, a day after the abductions.

Gunmen demanding the release of jailed Islamist militants had seized seven policemen and soldiers on a road between the Sinai towns of el-Arish and Rafah. Three of those abducted had worked at the Rafah border crossing, locals said.

"We will not open the crossing until the kidnapped soldiers are freed and the interior minister arrives to listen to our demands so that these attacks on us are not repeated," one of the protesting policemen said on Saturday.

Hardline Islamist groups in North Sinai have exploited the collapse of state authority after the overthrow of former President Hosni Mubarak in 2011 to launch attacks across the border into Israel and on Egyptian targets.

The protesting policemen called on President Mohamed Mursi, who is a moderate Islamist, to help free their colleagues.

The kidnappers have already released one policeman, security sources said. They had no word on the six remaining hostages. (Reporting by Yousri Mohamed; Writing by Shaimaa Fayed; Editing by Alistair Lyon)