Firefighters in Santiago de Compostela in the Galicia region are battling about 150 fires with 90 of them burning out of control.
Three people have been killed in the fires.
The Galician regional government deputy leader, Anxo Quintana, says 10,000 hectares (more than 24,700 acres) had been affected.
More than 1,200 soldiers have been sent to the region to help coordinate evacuations and to discourage arsonists.
About 50 firefighting airplanes and 6,500 firefighters have been battling to contain the blazes.
Environment Minister Cristina Narbona, has lamented that "the situation has worsened despite an increase in resources."
PM takes control
Mr Zapatero flew home from the Canary Islands to inspect the devastation for himself.
"There will be help for those affected" Mr Zapatero said after touching down not far from a fire which broke out near Santiago's airport.
He said his aim was to "assume responsibility" of the crisis.
The main opposition Popular Party had accused Mr Zapatero of showing "absolute indifference" to the fires.
Manhunt for arsonists
Noting the "very high number of fires" Mr Zapatero said police were working "with all resources at their disposal" to track down those believed to have set the fires.
He also saluted the efforts of the emergency services.
Civil guard head Joan Mesquida said the police were offering protection to anyone who wanted to inform on someone they suspected of arson and added that "various lines of investigation" were being followed up.
Five suspected arsonists have been arrested since Monday, one of whom has since been released.
Regional support
Neighbouring states rallied round to offer support to Spain with France and Italy each dispatching two fire-fighting aircraft.
Madrid had requested three Canadair fire-fighting airplanes, five helicopters equipped with special water buckets and 20 firetrucks.
Portugal is battling its own fires but announced it would send about 60 firefighters and 19 fire trucks to Galicia.
The Portuguese firefighters would remain in Spain for up to a week but Lisbon said it could not relinquish any aircraft.
Galicia suffers from massive forest fires each year but this year the government has had to deploy "unprecedented" resources.
Crisis looms in Portugal
Across the border in Portugal, more than 1,400 firefighters backed by 400 vehicles battled 21 blazes on Wednesday.
The National Fire Service said 14 of them were burning out of control.
Fires were reported in ten of mainland Portugal's 18 administrative districts as temperatures soared to 40 degrees celsius (104 degrees Fahrenheit).
Fueled by strong winds, a blaze near the southern city of Setubal, home to Portugal's third-largest port, was threatening dozens of homes, emergency services workers said.
About 100 firefighters backed by two water-dropping helicopters were at the scene of that fire.
Meanwhile a violent fire started Tuesday evening in an illegal rubbish dump in Greece, in the Corinth region, west of Athens.
It has damaged several hundred hectares of forest and arable land, firefighters said.
Some 300 hectares have already been burned and three villages were evacuated overnight as a precaution, said Prokopis Byzas, deputy mayor of Evrostinis where the fire broke out.