Lasso, an ex-banker who took office in May, last week scrapped planned incremental rises in gasoline charges, meant to eventually align with international costs, following pressure by indigenous and other organizations.
He opted instead to raise the price of gasoline extra, a higher octane gasoline that is Ecuador's most-used fuel, to a fixed $2.55 a gallon and diesel to $1.90 a gallon.
But unions and other groups want Lasso to freeze prices at lower rates and exempt sectors hit hard by COVID-19.
Gasoline costs have risen significantly since Lasso's predecessor, Lenin Moreno, began monthly increases in May 2020.
Police used tear gas in the capital amid small clashes with protesters, while officers on horseback blocked the entrance to the plaza that had been the marchers' destination.
Five police officers were injured and two members of the armed forces were being held by a community in northern Quito, but were unharmed, Defence Minister Luis Hernandez told journalists.
Thirty-seven people were detained for blocking roads, he added.
The Ecuador Confederation of Indigenous Nations said demonstrators had been injured but did not give a figure.