Egyptian President Mohamed Morsi called for a Dec. 15 referendum on a controversial new constitution Saturday, a day after an Islamist-dominated assembly rushed its passage and as his supporters jammed the streets in a massive demonstration organized by the Muslim Brotherhood.

In a speech Saturday to the constitution-writing assembly that liberal and other ­non-Islamist members had abandoned in protest, Morsi called for a “serious national dialogue” to resolve the political crisis sparked late last month when he decreed that he had near-absolute power in the name of speeding up Egypt’s democratic transition. Morsi has said his decree would be nullified once the constitution is adopted.

“In building our great nation, we have to overcome disagreements to move to build a great future,” Morsi said in his address. “Take into consideration that there are challenges for us in the future at home and abroad. We are capable as Egyptian people.”

In the streets around Cairo University, tens of thousands of moderate and more conservative Islamists — the same ones who had joined the revolution that ousted longtime president Hosni Mubarak nearly two years ago — cheered and waved Egyptian flags as Morsi announced a referendum they see as a moment of national triumph.

The other revolutionaries — members of the liberal and more secular segments of society who have been protesting against Morsi’s move all week in Tahrir Square — see it as one more step away from their vision of a progressive Egypt.

“Morsi put to referendum a draft constitution that undermines basic freedoms and violates universal values,” tweeted the leading liberal opposition figure Mohamed ElBaradei moments after the president’s speech. “The struggle will continue.”

In his address, Morsi sought to cast the constitution-drafting process as inclusive, despite the walkouts by secular, liberal, Christian and other non-Islamist members who felt thwarted by the Islamist majority. He noted that the assembly was elected by a democratically elected parliament and emphasized that his own powers would be significantly curtailed if the charter is adopted. He said that those assembly members who remained had sincerely struggled over questions of human rights. He also noted that the charter could be amended.

It is a document, he said, in which “the people are the source of authority.”

But Morsi’s decree, the hasty vote on the charter and now his decision to follow through with the public vote to approve it seem likely only to further galvanize Egypt’s normally fragmented opposition, which has already drawn improbable support from judges and other former Mubarak-era figures. Protesters have hit the streets by the tens of thousands over the past eight days, some calling for Morsi’s ouster, fearful he is becoming an Islamist version of Mubarak, but most urging him to abandon his decree and start the process of drafting the constitution all over again.

“He is calling for national dialogue? What dialogue? I mean, he refused dialogue concerning the constitution,” said Amro Suleiman, an official from the liberal Free Egyptians Party. “We feel the whole thing has been cooked. It is difficult to say what the reaction will be, but it will not be quiet.”

On Saturday, Morsi, who has voiced suspicion that the opposition is being hijacked by Mubarak loyalists, dismissed the idea of writing a new draft of the constitution.

“Egyptians are adamant about going forward to reach the objectives of the blessed revolution,” he said.

In the largest show of support since his election this summer, Morsi’s backers filled the wide, palm-lined boulevards around Cairo University on Saturday. They waved flags and portraits of Morsi, as well as elaborate flowcharts outlining conspiracies between the state media, liberal opposition figures and ex-Mubarak officials who they believe want to thwart the revolution.

Among the shoulder-to-shoulder crowd were moderate to liberal Muslims and others who say they think Morsi is simply doing what is necessary to keep the democratic transition on track in the face of near-constant opposition from the country’s top Mubarak-era judges. The judges dissolved the Islamist-dominated parliament this summer and had threatened to dissolve the constitution-writing panel, which is also mostly Islamist, before Morsi stepped in to rush the charter through.

“I admire Morsi’s decision and think it came at a suitable time for Egypt’s circumstances,” said Shaimaa el-Zoghby, 17, a student who came to the demonstration with her parents. “I know it’s not the best route we could move in. However, now it’s the only route to stop the anti-revolutionary forces who would do anything to regain power.”

Others in the crowd appeared mystified by those in the opposition, painting them as sore losers unable to accept that Muslim Brotherhood-backed candidates won in democratic elections.

“Why are they fearing the ballot box?” asked Hani Hamad, 32, an Arabic teacher from Cairo. “Why are they fearing democracy?”

Amer Shakhatreh contributed to this report.