In a move sure to be remembered during her expected run for mayor, the speaker of the City Council, Christine Quinn, cracked down yesterday on a fired council aide, Viola Plummer, barring her from the council chamber floor, with police on hand to enforce the rule.

A defiant Plummer and the council member she worked for as chief of staff, Charles Barron, left the chamber after three police officers and four security guards moved in on them before the official portion of the meeting.

Mr. Barron, a former Black Panther, rolled up his sleeves and stood as a buffer between Plummer and the police before she agreed to walk out. As he strode from the chamber with Plummer, Mr. Barron called out, “Christine, you’ll never be mayor.”

Reporters and television cameras swarmed the pair, who stopped on the steps of the rotunda of City Hall to say the ejection was racist and reminiscent of segregationist policies.

“This was a selective enforcement of the law so that they can continue the harassment, the retaliation, and the discrimination against not Viola Plummer and Charles Barron, but black people in general,” Mr. Barron said. “This is another form of Jim Crow-ism. They want us to sit in the back of the bus, in the balcony, anytime you are an assertive black, and that’s the problem in City Hall.” Plummer, who appeared visibly shaken by the standoff, said it should make no difference where she sits during a council meeting. What Ms. Quinn “thought she could do, and what she almost did, was to provoke me,” Plummer said. “I have a history, I have a history, of dealing with the likes of Christine Quinn.”

The speaker fired Plummer earlier this month after the aide promised in late May to end the political career of a council member, Leroy Comrie of Queens, even if it took “an assassination.” Plummer made the remark after Mr. Comrie abstained from supporting a proposal to rename a Brooklyn street after a deceased black activist, Sonny Carson.

The street name vote split largely along racial lines, with Ms. Quinn staunchly opposed to the idea, saying Carson had a history of making antiwhite statements. Plummer has defied the firing, continuing to work for Mr. Barron as his chief of staff even though she is no longer on the city payroll. Until yesterday, Mr. Barron had said Plummer had not faced any difficulties doing her job.

Ms. Quinn said yesterday that she planned to enforce a council rule that allows only council employees, government officials, and guests of the speaker to sit on the floor of the chamber during the official portion of a full meeting. “That rule will be enforced today as it has been enforced every other council meeting we have held since I have been speaker,” she said. “I have an obligation to make sure the business of this institution gets done.”

Council employees and others said they could not remember a time when the rule had been enforced so deliberately. When a contentious council bill is up for a vote, protesters or spectators occasionally are ushered to the balcony to watch the proceedings, but many said people barred by the council rule from sitting on the main floor during meetings often do so without any trouble. Plummer has sued Ms. Quinn in federal court for $1 million over the firing. The case is scheduled to go to trial on September 24.