CAIRO — Thousands of Egyptian protesters marched on the presidential palace and Cairo's downtown Tahrir Square on Tuesday to protest a contentious Islamist-backed draft constitution, after the country's Justice Ministry ordered a probe into allegations of widespread voting irregularities during Saturday's first round of voting on the document.
Since the country's current political crisis erupted more than three weeks ago, the opposition has kept the pressure on the government of President Mohammed Morsi with mass marches that at times have seen turnouts of hundreds of thousands.
Mr. Morsi's Islamist supporters have countered with rallies of their own.
Chanting "down with Brotherhood rule," and "your constitution is void," the opposition was making its first major street push since Saturday's round of the referendum on the constitution.
Preliminary results showed that 56 percent of voted cast "yes." The second round of voting is set for Saturday.
The opposition fears that large Islamist constituencies in rural and upper Egypt will increase the votes in favor.
Turnout was also low in Egypt's 150 diplomatic missions, which opened their doors for a half million Egyptian expatriates to vote.
Preliminary results showed stark difference between Egyptians in Arab Gulf countries and elsewhere. In Washington D.C., more than 70 percent of voters said "no," while in Saudi city of Jeddah, more than 80 percent voted "yes."
Islamists have suggested that passage of the constitution would give them a clearer mandate, but the opposition groups say the process has been rushed, turnout has been low and irregularities in the voting have been rife.
They insist that the constitution requires more than a simple majority, and many have called for the referendum voting to be repeated. The Brotherhood says the country's Elections Committee can adjudicate complaints.
The protests also follow closely on new blows in the conflict between Mr. Morsi and the judiciary. More leading judges announced a boycott of the second leg of voting, and the Prosecutor General Talaat Abdullah submitted his resignation just a month after Mr. Morsi appointed him.
Mr. Abdullah had come under fire from fellow prosecutors, who accused him of pressuring a judge not to release some 130 anti-Morsi protesters from detention.
Analysts were skeptical about Mr. Abdullah's resignation.
"The resignation of the prosecutor general is not innocent, and it is meant to rescue the referendum from becoming legally invalid and to push the prosecutors to oversee the vote," legal expert Nasser Amin said on the social network Twitter.
The Muslim Brotherhood issued a statement denouncing "forcing the prosecutor general to resign," calling it a "dangerous precedent."
The Brotherhood again described the opposition as "a group of thugs." It demanded that the country's Supreme Judicial Council reject the resignation.
Along with the prosecutors' protests, one prominent judicial body that did involve itself in the first round of voting, the State Council, said that it would boycott the second round to protest the alleged irregularities. The Council provided 1,500 of the 7,000 judges involved in the first round.
The vote on Egypt's post-revolution constitution comes against a backdrop of deep polarization that split the country's political forces into two camps: one led by Islamists including Mr. Morsi's Muslim Brotherhood group and ultraconservative Salafis, and the second led by the National Salvation Front, an alliance of liberal and left-leaning political parties and youth groups backed by Christians, as well as Muslims who are skeptical of the Brotherhood.