Major General Nabil al-Azabi issued the stern warning on Tuesday, a day before democratic activists and a banned Islamic group are due to hold a joint demonstration in front of parliament.
But despite the police warnings, the Kifaya movement, which has mounted several rallies against President Husni Mubarak since December, said it would go ahead with its Wednesday protest.
With Egypt under increasing pressure at home and abroad for greater democracy, Egypt has witnessed more daring pro-reform protests in recent months.
But the relative tolerance by police seems to be coming to an end.
Thousands of police clamped down on central Cairo to prevent a recent Muslim Brotherhood protest, the largest security action in the capital in years.
Major-General al-Azabi said that police would strictly enforce laws requiring groups to get permission for protests.
"It is permitted to express views, but if we are getting to the stage of [people] getting used to violating [the rules], then the principle is that legal regulations must be implemented," al-Azabi said. "Everyone should abide by the law."
Under Egyptian law, any rally or gathering of more than five people must apply for permission from the Interior Ministry at least three days in advance, and it is up to the ministry to decide whether to issue it.
"It is permitted to express views, but if we are getting to the stage of [people] getting used to violating [the rules], then the principle is that legal regulations must be implemented"
Major-General Nabil al-Azabi,
Cairo security chief
Nobody has requested permissions recently for protests, including for Wednesday's rally.
Al-Azabi would not say directly what action police would take to stop the planned rally, but he expressed growing impatience with the demonstrators.
Opposition activists should not be demonstrating "just for the sake of going out into the streets and repeating what they've already said", he said.
The opposition has representatives in the parliament to discuss their views, "not through gatherings in the streets that don't have legitimacy," he added.
Kifaya - whose name means enough in Arabic - has been calling for Mubarak to step down and for further reforms to be enacted.
Protesters want President Husni
Mubarak to step down
"The government is stronger than us, if they prevent us what can we do? If they arrest us, what's the problem? Each one has to bear the responsibility of his actions," George Ishaq, a Kifaya official, said.
About 200 members of the banned Muslim Brotherhood will also participate in the planned rally, although not officially in the group's name, Isam al-Arian, a leading Brotherhood member, said.
He said a total of about 500 people were expected at the demonstration.
The Kifaya movement has included some Islamist figures along with liberals and socialists. But the Brotherhood's participation marks a new cooperation between Kifaya and the group - probably Egypt's most powerful opposition movement.
"We are with all political national forces, our demands and goals are the same ... . Our goal is to keep up peaceful and gradual pressure on the regime, to achieve real, not cosmetic, reform," al-Arian said.