CAIRO, July 31 (Reuters) - Egypt has arrested 23 Egyptian and foreign Islamists pending investigation into accusations they were planning militant attacks and advocated the overthrow of the government, security sources said on Tuesday.

The sources said the men included an Egyptian cleric who had spent time in Iraq and two Yemenis, but gave few details on the identities of the remaining 20 detainees except to say they included non-Egyptian Arabs.

The arrests took place three weeks ago. The men were members of a Salafist group that follows a conservative purist brand of Sunni Islam, the sources said.

They gave no indication of what type of attack they believed the group was planning, but the arrests came as Egypt beefed up security measures on the Cairo underground rail network and at Cairo's international airport.

Fearing incidents similar to attacks in Algeria and Britain, authorities erected new police checkpoints with sniffer dogs on roads leading to and from the airport this month, and were searching all passengers rather than a random sample.

Senior police officials did not say whether they were acting in response to a specific security threat. Earlier in the month, Egypt also stepped up security on Cairo's underground rail network because of fears of possible attacks on stations.

Egypt's al-Masry al-Youm newspaper reported on Tuesday that authorities were also asking Egyptians in a Cairo suburb to report any large sales of nails, paint thinner and acid, especially to bearded men or foreigners, for fear they could be used in an attack.

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