Cairo's busy subway system has been on high alert since a hoax bomb threat was made at a station several days ago, police said Saturday.

Metal detectors and bomb sniffing dogs were deployed at all exits and entrances to the busy underground transportation network as a precaution, a police official said, speaking on customary condition of anonymity.

Vehicles and heavy trucks also were prevented from parking near subway stations around Cairo and its twin city, Giza, out of fear that they might contain explosives, according to the daily Al-Masry Al-Youm newspaper.

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The high alert came after an Egyptian man warned police about a bomb allegedly placed at a station north of Cairo's downtown on Wednesday. The warning though turned out to be a hoax, the official said.

No terrorism arrests were made, but police rounded up dozens of subway commuters - many who were arrested for possessing hashish, the official said.

Cairo 20-year-old subway system provides transportation for some 6 million Egyptians. It's known for its color-coded stations decorated with vibrant mosaics and small pharaonic statues.

The government plans to extend the subway soon to a location near the ancient pyramids of Giza