CAIRO — Just under one-third of voters turned out for the first stage of the referendum on a constitution meant to be a historic milestone in setting Egypt’s future — a showing critics say deepens doubts over the legitimacy of a charter that has already polarized the country.

The dismal showing also raises the question whether Egyptians have been turned off by the turmoil that has characterized the country’s politics throughout the nearly two years since the ouster of Hosni Mubarak’s autocratic regime.

Last Saturday’s voting took place in 10 of Egypt’s 27 provinces, including Cairo and the nation’s second largest city, Alexandria. Some 26 million voters were eligible to vote, but only 32 percent of them did. Voting in the remaining 17 provinces will take place Saturday.

Besides the low turnout, preliminary results show that the “yes” vote carried the first round only by a slim margin of 56 percent — hardly the resounding endorsement the Islamists were looking for to silence the increasingly vocal and united opposition that called on supporters to vote “no.”