BUENOS AIRES, Argentina — Tens of thousands of pro-government demonstrators marched through downtown yesterday in a show of support for an Argentine leadership facing a stiff challenge from rebellious rural growers.

But there was no sign yesterday that the farmers would lift roadblocks they have set up and resume shipments of beef and other food. On strike for almost three weeks, they have vowed to protest until at least today.

In the capital, union activists and others allied with the center-left government waved signs declaring “We are with Cristina,” to proclaim their solidarity with President Fernandez.

The president addressed the assembled masses from a stage placed in the highly symbolic Plaza de Mayo, where Argentines for decades have come both to protest and to honor their leaders.

“Let the roads clear so Argentines can have access to food,” Ms. Fernandez implored protesting farmers whose strike has led to supermarket-shelf shortages in one of the world’s major food-producing nations.

The president ignited the rural revolt last month with a plan to boost taxes on grain exports, especially soybeans, which have been a bonanza for growers cashing in on fast-growing Asian demand. Farmers rejecting the levies have blocked roads and suspended food shipments, leading to scattered shortages of meat, poultry, fruit, vegetables, and milk, among other products.

No one is known to have starved because of the strike, and restaurants in this diverse capital continue to buzz with business.

But the shortages and political turbulence have unsettled a nation that had enjoyed robust growth and relative stability since a 2001–02 economic meltdown. Last week, middle-class consumers took to the streets and beat pots and pans in support of the striking farmers.

Analysts call the almost three-week old crisis the strongest challenge to date to the five-year rule of Ms. Fernandez and her husband, President Kirchner. Mr. Kirchner eschewed almost certain re-election last year, stepping aside to let his wife, then a senator, seek office in his place.

Their leadership is widely credited with having helped bring Argentina back from the financial abyss. But the couple’s economic policies, including high export tariffs and price controls, have angered farmers and others in business. Critics call the couple’s leadership style authoritarian.

Yesterday, the president, in her fourth address about the strike in the past week, again refused to back down on her plan to boost export levies to as much as 45% on soybeans. Instead, in a speech appealing to nationalist themes, she called on farmers to allow food shipments to resume.

“It’s bad what they are doing,” the president said of the farmers’ roadblocks.

A day earlier, the president offered some concessions, including rebates and new transportation subsidies, to relatively small-scale farmers. But farm representatives labeled the measures insufficient.