Almost 100 G riders kicked off a monthlong campaign to increase service on the "forgotten stepchild" of the New York subway system, as Mr. Jeffries and others have called it.

"It's important to increase the intensity of the public campaign," Mr. Jeffries said, "to stress to the M.T.A. that G train service enhancements are absolutely necessary."

On June 25, the Metropolitan Transportation Authority board will meet to discuss systemwide service improvements. Mr. Jeffries, who organized the rally, intends to make sure the G is a top priority. In the coming weeks, G advocates will be writing letters, sending e-mails and corralling the support of elected officials in an effort to "convince the M.T.A. to do the right thing," as Mr. Jeffries put it.

In February, the agency announced a plan to increase the frequency of G service during off-peak hours, but these additions have been put on hold indefinitely.

"The M.T.A. is aware of these problems [on the G] but sometimes they just need to be reminded," said Cate Contino of the Straphangers Campaign.

Ms. Contino was one of a number of speakers who illuminated the unique woes of the G: truncated four-car trains, a lack of street transfers, long waits and a history of service reductions. Cuts to the G have occurred despite the route's expanding ridership, especially in the neighborhoods of Williamsburg, Greenpoint, Fort Greene, Clinton Hill and Bedford-Stuyvesant. In a recent rider report card survey, the G received a D+.

Joe Chan, of the Downtown Brooklyn Partnership, a nonprofit local development corporation, spoke of the need to grow downtown Brooklyn as one of the city's financial hubs—a project he says is hindered by the inadequacies of the G line.

The capacity crowd also included the president of the Pratt Institute, Thomas Schutte, as well as a representative from Brooklyn Borough President Marty Markowitz's office, City Council members, community leaders and residents of Brooklyn and Queens. It was Mr. Jeffries' rousing call and answer that received the audience's loudest response:

"What do we want?" he chanted.

"More G service!"

"When do we want it?"

"Now!"