CAIRO (AP) ― 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 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. Morsi’s Islamist supporters have countered with rallies of their own.

Chanting “down with Brotherhood rule,” and “your constitution is void,” the opposition made its first major street push since Saturday’s round of voting 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 because of irregularities. The Brotherhood says the country’s Elections Committee can adjudicate complaints.

The protests also follow closely on new blows in the conflict between 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 Morsi appointed him.

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 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,” said legal expert Nasser Amin on his social networking site 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 in protest at 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 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.

Liberals, Christians and others have criticized the Islamist domination of the constitution process, particularly the insertion of clauses that they say pave the way to a religious state and threaten civil liberties. They say the breakneck pace of its drafting and passage will only polarize the country further.

The Brotherhood counters that the passage of the constitution would be a much-needed boost for political stability.

Rights groups and opposition say they filed complaints of violations, including judges who they intentionally stalled the vote in constituencies anticipated to oppose it. They also say judges, whose supervision is required by law in Egyptian elections, were replaced by court employees in some districts to replace judges who boycotted the vote.

On Tuesday, Egypt’s Justice Ministry says it will assign judges to probe allegations of voting violations.

“This is the first time in the history of Egypt that judges are assigned to investigate vote violations,” a ministry spokesman said at a news conference.

Judge Mahmoud Abu Shousha, a member of the commission overseeing the referendum, rejected the charges of voting irregularities.

He said it was impossible to replace judges with court officials during the supervision, and that all stations stayed opened for four extra hours to accommodate the long lines, dismissing claims that some closed early. He said more staff will be recruited for the second round to speed up the process.

“We don’t know what to do with those who spread these lies,” he said at a news conference.

Many top Brotherhood officials have consistently characterized their critics as holdovers from the era of deposed president Hosni Mubarak. Most top judges are Mubarak-era appointees. The National Salvation Front is largely made of the Mubarak-era opposition, and Morsi’s critics also include some Islamists.

On Monday, Egypt’s Supreme Constitutional Court -- the country’s most prestigious tribunal that is at the center of the Brotherhood’s conflict with the judiciary -- denounced a statement by a Morsi aide in which it discussed the court under a “campaign” by “anti-revolutionary forces” to “overturn the gains of the revolution” against Mubarak.

Court spokesman Maher Sami accused Essam el-Haddad of “tarnishing” the court’s image and criticized him for writing the memo in English.

“The Supreme Constitutional Court is asking why the president’s aide chose to address the foreign media,” he said. He added el-Haddad aimed at “toppling the court’s reputation internationally” and that the “crime of spreading false and provocative news is punishable by law.”

El-Haddad, Morsi’s aide, denied the court’’s accusations and described them as “baseless.”