Youths in Catalonia and the Basque Country have daubed the slogan "tourists go home" on some buildings - just as foreigners flock to Spain on holiday, spending millions of euros.
A spokesperson for leftist Catalans behind the protests said today's model of mass tourism was impoverishing working-class people.
Leftist Basques plan to stage an anti-tourism march on 17 August in San Sebastian, during a major festival.
Semana Grande (Big Week) is a week-long celebration of Basque culture.
Regional officials say the protests are isolated - and insist that tourists are welcome. They deplore the acts of vandalism, and stress that tourism is a vital industry for Spain.
A record 75.6 million tourists visited Spain in 2016 - and Catalonia hosted 18 million of them, making it the most popular region.
Who is protesting, and how?
There has been some anti-tourist vandalism in Barcelona and Majorca - both Catalan-speaking - and most recently in San Sebastian, a tourist magnet in the Basque Country.
In one dramatic incident, several masked assailants attacked a tourist bus in Barcelona, near the football stadium.
The slogan "tourism is killing neighbourhoods" was daubed on the bus and one of its tyres was punctured. None of the passengers were injured.
The attack was claimed by Arran Jovent, a leftist youth movement linked to Popular Unity Candidacy (CUP), an anti-capitalist party campaigning for Catalan independence.
CUP tweeted a photo of the vandalised bus, with the message: "We show support for the youth organisation @Arran_jovent, we must combat the murder of barrios [neighbourhoods] with all means."
On its Facebook site, Arran has posted anti-tourism videos. One shows activists in Majorca holding a big banner next to yachts in a marina, letting off smoke bombs and throwing confetti inside a restaurant where customers are eating.
Elsewhere, the tyres of some tourists' rental bikes have been slashed.
Arran says it opposes a model of mass tourism that "wants us as slaves" and that "turns the country into an amusement park that only benefits the bourgeoisie and capital!"
The anti-tourism campaign has also sparked a heated debate on Twitter, under the hashtag #touristgohome.
An Arran spokesperson told the BBC that "we are anti-capitalist, we want to destroy the system - and the tourist industry is part of that system".
"Today's model of tourism expels people from their neighbourhoods and harms the environment - we've seen that all along the coast, with buildings everywhere.
He said Arran's protests were "not vandalism, but self-defence".
"Having to sign poor work contracts - that's violence - as is having to leave your neighbourhood and your support networks, because of tourism."
In the Basque Country, Ernai is a leftist youth movement acting in solidarity with Arran. Ernai emerged in 2013 from Basque nationalist groups who have campaigned for years on behalf of Eta prisoners.
Eta is disarming, after decades spent fighting the Spanish state, but Madrid refuses to negotiate with what it calls a "terrorist" group.
The Arran spokesperson said his group and Ernai were both "revolutionary movements" acting against "an oppressor Spanish state".
Arran has about 500 activists throughout Catalonia, and its campaign is attracting new members, he said.
So are the protests more political than economic?
In both regions the anti-tourism campaign is certainly coloured by nationalism.
There is resentment over tourism pushing up prices - especially apartment rents - for locals. But it is also about asserting national identity and fighting globalisation.
Catalans and Basques have a long history of struggle against Spanish political domination, going back to the Franco dictatorship and civil war.
A tweet from Katu Arkonada in San Sebastian showed the slogan "tourist go home" daubed on a restaurant.
Basque tourism has grown since Eta violence subsided. San Sebastian is now a gastronomic capital, with a winning combination of beaches, Basque cuisine and local colour.
Neighbouring Catalonia is in the tense run-up to a controversial referendum on independence in October.
The region's ruling coalition has vowed to declare independence immediately if a majority of voters back it. Opinion polls suggest a narrow majority wants to remain part of Spain.
Why have tensions surfaced in Barcelona?
Tourism is the number one problem for Barcelona residents, according to an opinion poll published by the municipality in June.
Despite Spain's stubbornly high unemployment, tourism was the top concern of residents (19%), ahead of unemployment (12.4%) and transport (7%). It is the first time tourism has come top in the regular poll.
Temporary mayor Gerardo Pisarello dismissed talk of "touristophobia". "I don't believe that Barcelona's residents reject tourism - rather they want it regulated," he said.
Loose regulation is blamed for the proliferation of cheap holiday accommodation in the city, such as Airbnb apartments.
Carol Olona, a BBC journalist from Barcelona, says the drive to cash in on tourism has pushed up prices in the city centre.
Many young Spaniards earn no more than €1,000 (£902; $1,176) a month, yet an ordinary apartment in central Barcelona can cost €800 or more in monthly rent.
Spain's El Periodico newspaper reports that many apartment rents in central Barcelona rose more than 10% in 2014-2016.
Ms Olona says many residents "have been struggling to find affordable rents, and moving to small places on the outskirts".
"But Barcelona doesn't have much space to grow - there is sea on one side, mountains on the other and the city is surrounded by urban areas."
In summer the daily influx of tourists from cruise ships has also fuelled resentment, she said.
Is there any official sympathy?
Yes, some. But it is tempered by exhortations to keep welcoming tourists, because of their economic value.
Arran wants a freeze on new hotel construction, more regulation of tourism and a model that "really respects the dignity of working people".
In Majorca, Balearics Deputy Prime Minister Biel Barceló said he shared the concerns about today's mass tourism and the "unbalanced" model. But he condemned Arran's methods.
A leading Basque tourism official, Denis Itxaso, said "concentrations of tourists" were inevitable in some places, but stressed that tourism was a vital source of income.
"Mind you don't play around with the hen that lays the golden eggs," he warned.
Ernai then mocked him in a tweet, showing him as a hen, with the message "Your golden eggs are the misery of young people!"