The strike began in Barcelona last week and spread to Madrid at the weekend as drivers blocked main thoroughfares, demanding action from the government.
Strikes or partial stoppages were also called in Valencia in eastern Spain, Zaragoza and Bilbao in the north, and Seville in the south.
Representatives of taxi associations held talks Monday with the public works ministry. But these ended without a deal and the negotiators said the work stoppage would continue.
The strike was sparked after the Spanish government appealed a ruling by the Barcelona authorities that limited the number of licences for Uber-style services.
- Taxi strike against Uber spreads across Spain
- Madrid taxis join Barcelona strike against 'unfair' Uber and Cabify
- IN PICS: Uber and Cabify halt Barcelona services after striking drivers assaulted
Taxi driver federations want the authorities to strictly enforce the legislation under which there should be 30 traditional taxis for each VTC (Tourism Vehicle with Chauffeur).
They argue their licences are much more expensive than those for VTCs.
Today there are only five traditional taxis for every VTC, they say, warning that they will extend their action to blocking ports, airports and even the border with France if Madrid fails to act.
VTC drivers have in turn criticised their traditional taxi peers, charging that they were attacked in Barcelona last week.
"We sincerely hope that the state does not give in to blackmail from violent people," said Unauto, the platform which represents Uber and Cabify, its Spanish competitor.
In Barcelona, Spain's biggest tourist attraction, hundreds of black and yellow taxis parked for a third day on the Gran Via, one of the city's main roads.
Some drivers camped out there, sleeping on mattresses on the ground, in tents or in their taxis.
In Madrid, they also stopped and blocked the main Paseo de la Castellana thoroughfare.
"It's a big sacrifice for us to be here, it's peak season, it's full of tourists and we could be making good money but we have to say enough," said Antonio Ramirez, a 38-year-old cab driver in Barcelona.
"If they start giving VTC licences here, there and everywhere, then there will be as many VTCs as taxis and there's not enough business.
"Many of us have taken out a loan to get a taxi licence," he added.
But Jose Antonio Robles, who recently turned to driving a VTC after 40 years as a taxi driver on the southern Costa del Sol, was angry.
Taxi drivers "want to try and forcefully recover their monopoly, hijacking the citizen's opinion and forcing them to consume what they don't want," the 63-year-old said from Malaga.
He added taxis should improve their offering if they wanted to attract customers, by for instance offering baby seats, the possibility to pay by American Express, and more generally have available card payment terminals.
The public works ministry said in a statement it had offered to implement a decree in September "with which we will manage to bring stability" and aim to enforce the one-30 ratio.
Taxi federations said this was too vague.
"Taxis won't move from the Castellana until there is a clear statement of intent from the ministry," tweeted Elite Taxi Espana, a taxi federation present in the negotiations.
By AFP's Daniel Bosque