Protest was held after Spain's Deputy Prime Minister Carmen Calvo indicated that the pardons of the Catalan separatists were "close" - an issue that has bitterly divided the Spanish society.
Tens of thousands of Spaniards protested in Madrid against government plans to pardon 12 Catalan politicians who were convicted over the region's failed independence bid in 2017, a move the demonstrators see as a threat to national unity.
Madrid's Municipal Police force estimated 126,000 people took part in the protest on Sunday while the National Police put the figure at 25,000.
Spain's Deputy Prime Minister Carmen Calvo gave the clearest indication yet that the pardons of the Catalan separatists were "close" in an interview published on Sunday with La Vanguardia newspaper, but the issue has bitterly divided Spaniards.
Around 63 percent of Spaniards oppose granting the pardons while some 25 percent backed it and about 6 percent were indifferent, according to a poll published on Sunday for niusdiario.es, an online newspaper.
"We should stop this now because for a pardon you must show contrition, and the separatists will not show that," Carlos Bandecha, 47, a businessman, said at the rally, where many protesters waved red and yellow national flags.
Opposition lawmakers from the conservative People's Party (PP), the far-right Vox party and the centrist Ciudadanos joined the rally in Madrid's central Plaza Colon.
Asked when Spain's centre-left government might approve the pardons, Calvo told Vanguardia: "Soon, they are close. After the Supreme Court report arrives, we will immediately follow up on them. They should arrive soon at the cabinet."
Spain's Supreme Court said in a non-binding report last month that it opposes potential government pardons for the Catalan separatist leaders.
It sentenced nine Catalan separatist leaders in October 2019 to between nine and 13 years in jail on sedition charges for organising an unauthorised referendum on independence and issuing a short-lived unilateral declaration of independence.
Three others were convicted of disobedience but not jailed.