BUENOS AIRES — After days of intense clashes outside Argentina’s Congress that left scores injured and led to at least 70 arrests, lawmakers on Tuesday approved a contentious bill tightening pension and social welfare benefits. The bill’s passage was a victory for President Mauricio Macri, but resistance to the changes has put him on the defensive.

The widespread opposition to the measure has called into question Mr. Macri’s ability to accelerate the pace of change he says is necessary to put Argentina’s economy on a solid footing after his coalition’s decisive victory in midterm elections.

“I didn’t vote for him so he could take money from my pocket,” said Cecilia Fernández, 68, one of thousands who took to the streets late Monday to bang pots and pans to protest against the bill.

The first sign that the bill would face stiff resistance came last week, when some opposition lawmakers joined large protests that broke out in downtown Buenos Aires, the capital. Lawmakers supporting the bill postponed debate.