Other international organizations known for monitoring polls, the OECD and the United Nations, have not posted announcements they would participate.
Polling in the referendum is split between two days.
Over 6500 stations will welcome up to 26 million eligible voters in the first round of balloting, with the military and police working together to ensure security and proper procedure, MENA said.
Ten provinces, including the highly populous ones of Cairo and Alexandria, vote until 9:00 p.m. Saturday, MENA reported, after having opened at 8:00 a.m. Seventeen more provinces vote in a week on December 22, rounding out the referendum.
Long lines of voters cued up ahead of poll openings, and turnout has been high, the electoral commission said, leading it to extend voting by two hours. It was originally scheduled to end at 7 p.m. Polling has gone smoothly so far, according to the election commission.
The rocky road to the referendum began when judges threatened to shut down the assembly tasked with drafting the constitution. President Morsy then issued an edict in late November declaring all of his past and present decisions immune from judicial review until the holding of the constitutional referendum.
He also sacked the head of the judiciary. The judicial system has many in its ranks who are loyal to former autocratic President Hosni Mubarak.
The Islamist president's opposition saw the exceptional move as a grab for dictatorial powers and poured into the streets, converting Tahrir Square in central Cairo back into a the center of public discontent it had been during the uprising that brought down Mubarak.
The president has since dropped his provocative decree going forward, but the situation has remained tense, and violence has continued.
In response to violent clashes, Morsy has given the military the authority to make arrests during the electoral run-up.
Morsy's Islamist allies rushed the drafting of the constitution to completion, which some saw as a tactic to allow him to drop his controversial edict more quickly. Others feared it to be another grab for power. Non-Islamist assembly members quit the process, which served to increase suspicion against the Islamists.
The outcome of the election and the unrest associated with it are important to the stability of volatile North Africa and the Middle East -- where Egypt is a key player -- and the situation is being watched closely around the world.