MADRID – Tens of thousands of protesters on Sunday called for the ousting of Spanish Prime Minister Pedro Sánchez and denounced his proposal to begin talks to try to ease tensions around the secessionist conflict in Catalonia .

The protest – which took place in one of the main squares of Madrid and organized by right-wing parties under the slogan "For a united Spain, elections now!" – was the biggest demonstration against Mr. Sánchez since taking over his minority socialist government in June.

The government's proposal last Tuesday to appoint a rapporteur for talks between political parties to deal with the crisis of Catalan independence has galvanized the opposition, which considers it a betrayal and capitulation to pressure from Catalan separatists.

The protest took place on Sunday, two days before the start of the historic trial of 12 Catalan separatist leaders, which may also provoke emotions in the face of a territorial conflict that remains at the center of Spain's political agenda. The accused are accused of rebellion and other crimes stemming from the declaration of independence of their region in the fall of 2017.

On Sunday, Pablo Casado, leader of the conservative opposition People's Party, told protesters, many of whom waved Spanish flags, that Sánchez was negotiating "under the table" with separatist Catalan politicians. He called on the prime minister to "immediately stop the dialogue with those who want to divide Spain" and to hold quick legislative elections.

Before the demonstration, Sánchez had said that he was working "to solve a state crisis compounded by the Popular Party while he was in power," especially during the secessionist turmoil of 2017.

In October of this year, the crisis in Catalonia broke out when a separatist majority of legislators in the Catalan regional parliament unilaterally declared its independence, which prompted the central government of Madrid to impose a direct government on Catalonia. Their statement follows a referendum on independence won by secessionists.

The Spanish courts had declared the referendum unconstitutional even before it was held, but the vote still took place between clashes between the Spanish National Police and the voters going to the polls.

Most of the 12 defendants who appeared before the Supreme Court were members of the Catalan government that organized the referendum. Nine of them have already spent more than one year in prisonafter being refused the deposit.

Mr. Sánchez has a tenuous in power, at the head of a socialist party that controls only a quarter of the seats in Parliament. This month, he hopes to get approval of his national budget by Parliament, which would require to retain the support of legislators of Catalonia, who were part of the smaller parties that helped him to win unexpectedly. the position of president last June.

These parties declared that their vote for the budget was conditioned to the Catalan talks, including the question of independence, which the government will not include.

The Sunday demonstration in Madrid brought together the leaders of the three parties most fiercely opposed to the Catalan independence movement.

Mr. Casado was accompanied by Albert Rivera, from the center-right party Ciudadanos, and Santiago Abascal, founder of Vox, a far-right party that has electoral breakthrough last December at an election in southern Andalusia.

Abascal told the crowd that the central government should put Catalonia under direct control and take steps to "quell" the independence movement.

Recent polls have shown that the People's Party, Ciudadanos and Vox could win the majority of the seats in Parliament if elections were held, thus allowing them to replace the socialist government.