Basic facts on Egypt's constitution referendum

— Saturday's vote takes place in 17 of Egypt's 27 provinces, with some 25 million people eligible to vote. Polls open at 8:00 a.m. (0600 GMT) and close at 7:00 p.m. (1700 GMT), although authorities often extend voting for several hours.

— Preliminary results will likely be known late Saturday or early Sunday, as observers compile results announced at each polling station. Official final results are not expected for several days afterward, but such preliminary results have proven accurate in past elections.

— In the first round, held on Dec. 15, preliminary results showed a low turnout of 32 percent, with 56 percent voting "yes" for the constitution in voting that took place in 10 provinces, including the two biggest cities Cairo and Alexandria.

— Among the areas voting is Cairo's twin city of Giza, capital of the province of the same name, Egypt's third most populated with nearly 4 million registered voters. Also voting will be Nile Delta provinces in which Islamists who back the charter enjoy large constituencies, such as Beheira with 3 million registered voters. The "no" vote could be stronger in the three Suez Canal cities — Port Said, Ismailia and Suez — and the Nile Delta province of Menoufia.