CAIRO, Oct 24 (Reuters) - Egyptian authorities detained 13 students affiliated with the Muslim Brotherhood, the country's most powerful opposition group, after clashes with security men at a university campus on Wednesday, Brotherhood sources said.

They were protesting against alleged vote rigging in student elections at Cairo's Ain Shams University on Monday. The university authorities disqualified all Islamist candidates, common practice in Egyptian university elections.

The students had gathered in front of university offices to collect signatures and read out a prepared statement when security men in plain clothes attacked them, Brotherhood students said.

An interior ministry spokesman said he had no knowledge of the detentions or the university clashes.

The government calls the Brotherhood an illegal organisation and often detains its members without charge, but the Islamist group has one fifth of the seats in parliament and operates openly -- within strict limits.

Amr Sherif, a photographer covering the protest for the opposition newspaper al-Destour, told Reuters he was taken to hospital after policemen armed with sticks beat him and took the memory card from his digital camera.

Brotherhood sources also said security men used improvised percussion grenades and Molotov cocktails.

In the Sharkia province northeast of Cairo, police also detained four Brotherhood members on Wednesday over clashes 11 days ago, security sources said. The clashes broke out when a dispute over prayers marking the Muslim feast of Eid el-Fitr turned violent.

Brotherhood officials said the authorities are now holding about 150 members, many of them without charge. The number is down from several hundred earlier in the year.

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