Mariano Rajoy is expected in Barcelona, the Catalan capital, at 11am to show support for his Popular Party's campaign ahead of a vote next month.
His visit comes two weeks after he dismissed Catalan leader Carles Puigdemont, his government and the parliament, suspended the region's autonomy and called for new elections there on December 21st.
On Saturday, hundreds of thousands of Catalans protested the jailing of regional officials for their push for independence from Spain, which has left the country mired in a political crisis.
The demonstrators gathered on an avenue next to the regional parliament building waving Catalan independence flags and chanting "Freedom!" while some held up banners announcing: "SOS Democracy".
Rajoy, who is due to attend a presentation by a party candidate at a station hotel, will not make a public appearance in the city.
The protest followed the release from jail of the region's parliament speaker -- one of dozens of lawmakers sacked by Madrid last month – after posting 150,000 euros bail.
Children in riding helmets climbed castells -- the region's traditional human towers -- as others held placards bearing caricatures of some jailed lawmakers.
Barcelona municipal police put turnout for the march at some 750,000 people as crowds stretched for more than 15 blocks along the boulevard.
The Catalonia crisis has caused concern in the European Union as the bloc deals with Brexit and uncertainty over the fate of the region's 7.5 million people. More than 2,400 businesses have moved their legal headquarters elsewhere.
On Wednesday, a general strike called by a pro-independence union caused travel chaos, blocking 60 roads and train lines including Spain's main highway link to France and the rest of Europe.
Since lawmakers in Catalonia -- a wealthy region with its own language and distinct culture -- declared independence on October 27th following a banned referendum, pro-separatist officials have come under huge pressure from Madrid.