Polls suggest neither side has a firm grip on Argentines' sympathies.
Fernandez easily won re-election just a year ago with 54 percent of the vote but saw her approval rating fall to 31 percent in a nationwide survey in September by the firm Management & Fit. The poll of 2,259 people, which had an error margin of about two percentage points, also said 65 percent of respondents disapproved of her opponents' performance.
Crime is the biggest concern for many of her critics.
Newspapers and television programs provide a daily diet of stories about increasingly bold home invasion robberies, in which armed bands tie up families until victims hand over the cash that many Argentines have kept at home since the government froze savings accounts and devalued the currency in 2002. The vast majority of the crimes are never solved, while the death toll is rising.
Inflation also upsets many. The government's much-criticized index puts inflation at about 10 percent annually, but private economists say prices are rising about three times faster than that. Real estate transactions have slowed to a standstill because of the difficulty in estimating future values, and unions that won 25 percent pay hikes only a few months ago are threatening to strike again unless the government comes up with more.
The phrase "Cristina or nothing" was stenciled on buildings surrounding the Plaza de Mayo.
Demonstrators held up signs accusing the president of arrogance. While some featured a lengthy list of demands, others simply said "basta" — enough.
Associated Press writers Michael Warren, Almudena Calatrava and Emily Schmall in Buenos Aires; Frances D'Emilio in Rome; Jorge Sainz in Madrid; and Luis Andres Henao in Santiago, Chile, contributed to this report.
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