Egypt's state TV says Vice President Mahmoud Mekky has resigned.
Mekky's Saturday resignation was announced with more than five hours to go of voting in the second and final phase of a referendum
on a disputed, Islamist-backed constitution. Mekky, a career judge, has said he intends to quit once the charter is adopted. The new constitution eliminates the post of vice president.
However, a statement by Mekky read on state TV hinted that the motive of his hurried departure could be linked to the policies of Islamist President Mohamed Morsi.
He said he first submitted his resignation last month but events forced him to stay on.
"I have realized a while ago that the nature of politics don't suit my professional genesis as a judge," he wrote.
Saturday's vote is taking place in 17 of Egypt's 27 provinces with about 25 million eligible voters. The first phase on Dec. 15 produced a "yes" majority of about 56% with a turnout of some 32%, according to preliminary results.
There has been little indication that the result of the vote will end the political crisis in which the country is mired.
For some supporters, a "yes" vote was a chance to restore some normalcy after nearly two years of tumultuous transitional politics following Egypt's 2011 revolution, or to make society and laws more Islamic. Opponents saw their "no" vote as a way to preserve the country's secular traditions and prevent Morsi and his Muslim Brotherhood group from getting a lock on power.
Voters reflected the schism over the constitution – and over Morsi himself.
"I came early to make sure my 'no' is among the first of millions today," oil company manager Mahmoud Abdel-Aziz said as he waited in line outside a polling station in the Dokki district of Giza, Cairo's twin city on the west bank of the Nile.
"I am here to say 'no' to Morsi and his Muslim Brotherhood," he said.
Another Giza voter, Sahar Mohamed Zakaria, had a different take on Saturday's vote.
"I'm voting 'yes' for stability," Zakaria, an accountant and mother of three, announced.
Preliminary results for the second round are expected late Saturday or early Sunday. The charter is expected to pass, but a low turnout or relatively low "yes" vote could undermine perceptions of its legitimacy.