Rajoy was 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 21.
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 (HK$1.36 million) bail.
Barcelona municipal police put turnout for the march at 750,000 people as crowds stretched for more than 15 blocks.