Police in Alexandria have fired teargas rounds at Islamist supporters of Egyptian President Mohammed Mursi.
The confrontation comes on the eve of a divisive referendum on the new draft constitution.
The Muslim Brotherhood called the rally to decry the violence between Islamists and secular Egyptians last week.
A well-known conservative Muslim preacher was trapped in his mosque for 14 hours as rivals attacked one another with clubs and swords.
Sheik Ahmed el-Mahalawi denounced opponents of the new constitution as “followers of heretics.”
On the streets opinion is polarised.
“This one million man demo today is to say “yes” to the constitution. Secondly it is to show the value of mosques and teachers for Muslims. What happened last Friday, attacking the mosque, is not accepted by Muslims and non-Muslims,” said one man.
Across Egypt preparations are underway for the second part of the referendum.
The ‘Yes’ campaign narrowly won the first vote on December 15.
Those opposed to the president believe the draft constitution is pro-Muslim and fails to protect the rights of all Egyptians.
The constitution must be in place before elections can go ahead within two months, but the bill has exacerbated the spilt between Islamists and secular groupings.
The country has been in a state of chaos since Mubarak was disposed in February 2011.