Less than 30 seconds into the opening, the exchange suspended trade for half an hour under circuit-breaker rules designed to cushion falls. Share prices resumed their decline after the suspension ended.

Acting exchange chairman Mohamed Abdel Salam told Al Arabiya television trading would continue to the end of the session once it resumed after the suspension.

Under circuit-breaker rules, if the index falls a total 10 percent after the suspension ends it is up to the chairman to decide whether to allow trading to continue. The session was set to close at 1130 GMT.

"I think the decision to keep trade open is quite good," said Amr El Feky at Cairo Capital Securities. "As happened in Japan, they allowed the session to react normally. I think this will be positive next week ... We already had the shock and we should start thinking where to begin our investments."

Major Egyptian shares plunged when the exchange reopened. Orascom Construction Industries, Commercial International Bank, EFG-Hermes and Telecom Egypt all fell by the 10 percent limit for individual stocks.

Shares in Orascom Telecom, which has a substantial portion of its operations outside of Egypt, traded in a broad range, rising at one point by as much as 7 percent.

By 0933 GMT, it was trading 0.6 percent lower than its Jan. 27 close.

The exchange suspended a number of companies until further notice until their ownership and financial positions were clarified. These included Ezz Steel, Amer Group, Heliopolis Housing, Mokhtar Ibrahim and National Societe Generale Bank (NSGB).

Ezz Steel Chairman Ahmed Ezz, who was a top official in ousted president Hosni Mubarak's party, is being investigated for graft. He has been detained since Feb. 17 pending trial on suspicion of squandering public funds.

The stock exchange requested disclosure of assets linked to ongoing investigations, it said in a statement.

The exchange had suspended trade on the whole market for a half hour after the EGX100 fell 7.0 percent at the open.

Both the EGX100 and the benchmark index were 8.7 percent lower by 1017 GMT, with the EGX30 at 5,155 points.