Thousands of protesters packed Madrid today in a fierce backlash against Spanish Prime Minister Pedro Sanchez and his planned talks with Catalan separatists. 

Demonstrators waving Spanish flags filled the Plaza de Colon in the city centre in the largest protest Sanchez has faced since taking office last year. 

Some 45,000 people turned out in a show of force against the Socialist leader, demanding new elections and chanting: 'Spain! Spain!' and 'We want to vote!'

Sanchez, who relies on backing from Catalan nationalists and other minority parties to stay in power, plans to appoint an intermediary to resolve the dispute but was accused of 'treason' and caving in to the nationalists. 

Recent polls have shown the conservative Popular Party, center-right Ciudadanos and far-right Vox could together win a majority of seats in parliament if elections were held, allowing them to replace the Socialist government.

'The time of Sanchez's government has ended,' Popular Party leader Pablo Casado told reporters before the protest. 

Some banners at the rally read 'Sanchez, liar' and 'Spain is not negotiable and cannot be sold'.

Sanchez is also under pressure from Catalan nationalists, who want a legally valid referendum on independence which Madrid has refused to allow. 

Ana Puente, a 73 year-old retiree who attended the gathering, said she was protesting for a 'united Spain'.

'The government is giving many things to supporters of Catalan independence and is going to break Spain apart,' said Raquel Garcia, 76, who was carrying a Spanish flag.

Sanchez told a separate rally in the city of Santander: 'The government works for the unity of Spain and that means to unify Spaniards and not to put one against the other as the right is doing today in Colon square.' 

The protest took place two days before the start of the trial of 12 Catalan independence leaders, who face up to 25 years in prison for their role in 2017's failed secession bid from Spain.

A referendum in October that year was declared invalid by Madrid and separatist leader Carles Puigdemont fled into exile in Brussels.     

The government faces a key vote on Wednesday on its 2019 budget proposal, which will likely fail without the support of Catalan parties. 

But those parties have said their vote for the budget is conditional on the Catalan talks including the issue of independence, something the government will not include.

Failure by parliament to approve the budget bill could prompt a snap election before the next scheduled vote in 2020.