As Wednesday’s clashes began, Vice President Mahmoud Mekke offered a compromise that seemed to go nowhere. Mr. Mekke proposed that both sides agree in advance on a package of amendments to the text of the draft constitution to build more support for it before the Dec. 15 vote.

“All the political forces objecting to some articles in the constitution are welcome to provide suggestions or concepts about the articles,” he said, suggesting that through “calm dialogue” both sides could agree on revisions that would be approved by the future Parliament after ratification.

The vice president, however, did not suggest any means to overcome the lack of trust in the Islamist leaders among the secular opposition, or how to persuade liberals to back down from their vow not to negotiate until Mr. Morsi relinquishes the temporary expansion of his powers and cancels the referendum.

Mohamed ElBaradei, the former United Nations diplomat, was chosen Wednesday as coordinator for the newly unified secular opposition. He urged Mr. Morsi and his allies to “see what is happening in the Egyptian street, the division, the polarization. This is something that leads us to violence and worse.”

“The ball is in his court,” Mr. Mr. ElBaradei said at a news conference in which he threatened a general strike or other action to try to stop the referendum. “Bullying will not yield any results for this country.”

“The people of Egypt will be gathering everywhere,” he added. “We will not finish this battle for our freedom and dignity until we are victorious.”