Hundreds of journalists rallied on the steps outside the union headquarters, chanting "Journalism is not a crime!" and demanding the dismissal of the country's Interior Minister Magdy Abdel-Ghaffar.
The protests are the latest in a series of demonstrations against the government of President Abdel-Fattah el-Sissi, which has banned virtually all protests and carried out a wide-ranging crackdown on dissent.
Police severely restricted access, banning non-union members from entry, as well as some residents and people who came on work errands to the surrounding area. Foreign journalists were allowed entry only after approval by several levels of officers, up to the rank of general.
"There are thugs here threatening us, and the police don't want us to enter for the meeting because they know we'll condemn the Interior Ministry," said journalist Ahmed Bakr, who was allowed into the building's street eventually.
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Several dozen counterdemonstrators and el-Sissi supporters showed up at either end of the blocked-off street, blasting patriotic songs, chanting "long live Egypt" and insulting union members, who responded by calling the police "thugs."
"Those who made this mistake should be held accountable," he said from the syndicate's steps. "We are here to keep the dignity of our union, and show support and unity against what happened."
Since the ouster of Egypt's Islamist President Mohammed Morsi in 2013, the government of army chief-turned-president el-Sissi has clamped down on political demonstrations, mainly by Islamist opponents demanding Morsi's return.
Hundreds of protesters have been killed and thousands detained over the past three years, and a draconian anti-protest law has virtually banned all street demonstrations without prior police permission.