Nine policemen are alleged to have assaulted the doctors in a state hospital in Cairo's northern district of Matareya on January 28 after they refused to forge a medical report.
The prosecutor's office called the nine policemen in for questioning but released them on Thursday.
The head of the doctors' syndicate, Hussein Khairi, told AFP: "Our demands, which are very fair, are the safety of doctors while performing their job and that the attackers be taken to court."
"The interior ministry are thugs," the protesters chanted, standing 500 metres (yards) away from four armoured police vehicles. There were no clashes.
Heart doctor Ahmad Sweif, 35, had travelled to the protest with his wife from the Nile Delta, north of the capital.
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"I came to send the message that the dignity of doctors and all Egyptian people cannot be violated," he said.
Doctor Nagat Abdelghani said: "It's a shame that the police who are supposed to protect us are the ones attacking us."
An interior ministry spokesman did not respond to a request for comment.
On Tuesday, a court sentenced a policeman to eight years in jail on charges of beating a veterinary surgeon to death while in custody in the northeastern town of Ismailiya.
Five years ago, 18 days of protests -- motivated largely by anger over police brutality -- led to the ouster of longtime dictator Hosni Mubarak.