Seven international human rights organisations have sharply criticised the Egyptian authorities for extending the detention of a leading democracy campaigner.

Saad Eddin Ibrahim, a professor of political sociology at the American University in Cairo, has been held in custody for more than two weeks.

Egyptian prosecutors remanded Mr Ibrahim and his assistant Nadia Abd al-Nur on Thursday for another 15 days in jail.

They were arrested on 30 June, on charges of collaborating with foreigners to undermine Egypt's stability.

Vague charges

Mr Ibrahim is also director of the Ibn Khaldoun Centre for Development Studies, a think-tank which receives some foreign funding. It has now been closed and sealed off by police.

Egyptian security sources said Mr Ibrahim was accused of having links with a foreign military agency and an Israeli diplomat.

However, the seven rights groups, including Amnesty International and Human Rights Watch, said the charges against Mr Ibrahim were so lacking in detail that they did not allow his defence to challenge the legality of the detention order.

In a joint statement, the groups said Egypt's "international obligations require it to respect the rights of detainees, which include to be informed promptly and in detail of the charges against them".

The American University in Cairo also condemned Mr Ibrahim's arrest as a threat to academic freedom in Egypt.

Mr Ibrahim was initially alleged to have received $220,000 from the European Commission without government permission, to produce a pro-democracy documentary on the election process in Egypt.

Eleven others at the Ibn Khaldoun Centre are reported to have been detained since Mr Ibrahim's arrest.

The following organisations signed the joint statement:

• Amnesty International
• Article 19
• Euro-Mediterranean Human Rights Network
• Human Rights Watch
• International Federation of Human Rights
• Observatory for the Protection of Human Rights
• World Organisation against Torture

Mr Ibrahim has in the past made allegations of vote-rigging, which the government strongly rejects.

On Thursday, prosecutors said he was questioned about his relationship with the Israeli Embassy. Egypt signed a peace treaty with Israel in 1979, but their relations remain strained.

Mr Ibrahim also was questioned on another sensitive issue in Egypt: his think-tank's reports on the sectarian strife involving Muslims and Coptic Christians.