The crowd gathered Thursday, chanting slogans, waving banners, and banging pots and pans, the BBC reported.
They told the BBC they were angry about a proposed bill that would allow voters to choose the magistrates who would select judges. This, they said, would politicize the judiciary.
Government officials may be concerned the protests might strengthen the opposition during an election year, the BBC said.
Lorenzo Velazquez, 51, of Buenos Aires held a sign reading, "2015 Sin Cristina," referring to the year President Cristina Fernandez' term ends.
"I came to see if the Kirchners will leave ... and that peace returns so that we can be a united country once again," he said.