Officials said the wife and daughter of a police inspector were arrested on suspicion of gunning down Isabel Carrasco, conservative Popular Party (PP) leader of the provincial council in Leon, northern Spain.
Half a dozen police covered the body of Carrasco with a white sheet at the scene on a pedestrian bridge over the Bernesga River in the university city, media photographs showed.
Police rolled her body away on a stretcher and began dusting the bridge for fingerprints, a photographer with news agency AFP saw.
"She has died. She was shot several times and she died," a Popular Party spokeswoman told AFP after the late-afternoon shooting.
"We thought it was firecrackers," a pair on the scene told Spanish daily El Pais shortly afterwards.
"At that time there were people everywhere, and kids playing with a lot people walking their dogs."
Carrasco, 59, was pronounced dead at 5.20pm, the paper reported.
Spanish daily El Mundo reported that both a mother, 55, and daughter, 35 — the wife and child of a police chief inspector from the Spanish town of Astorga — had been arrested over the shooting.
They were identified by a retired policeman who was a witness on the scene, according to El País.
"It seems that the daughter was fired yesterday from the council where she worked as an engineer," an interior ministry spokesman told AFP.
"For that reason, everything indicates that it was personal vengeance," he said.
"The police are carrying out tests to find out who fired the shots," he added.
Police on Tuesday began searching for the weapon, believed to be a low-calibre revolver.
On Tuesday, El Mundo quoted sources from inside the PP as saying the daughter had recently lost a labour dispute with the provincial council.
Those sources said the result was definitely handed down a few days ago, and would see the daughter paying back money earned while working for the council on a self-employed basis since losing her job with them in 2011.
Police say this is one possible motive for the crime, although this is yet to be definitely established.
The killing shocked a country unused to such acts since the Basque separatists ETA announced an end to violence. Numerous PP officials were assassinated in the 1990s and early 2000s in killings blamed on ETA, which declared a definitive end to violence in October 2011.
Prime Minister Mariano Rajoy and other politicians from the ruling party and the main opposition Socialists said they were cancelling official engagements.
Rajoy said on Twitter he was "dismayed by the murder of Isabel Carrasco".
"My condolences and support to her family and friends. Now is the time to be united," he wrote.
"The Popular Party expresses its deepest sorrow and announces the suspension of all acts planned for today," the ruling party said in a statement.
The Socialist Party, too, issued a statement expressing its condolences over the killing and suspended its campaign events on Monday.
A lawyer by training, Carrasco was born in the province of Leon and held various provincial and regional posts, according to her biography on Leon city hall's website.
She had led the Leon provincial council since 2007.
The local newspaper Diario de Leon described her as a "powerful character".
"Despite her small size, her very strong character and the way she took the reins of power caused her to be known as the 'super-delegate'," it wrote.
"It was precisely this character and the power she accumulated that earned her numerous political rivals."
Besides being head of the Leon provincial council, Carrasco was also leader of the PP for the Leon province.
In 2010, Carrasco was reported to have had 13 different jobs within Spain’s ruling Popular Party and other companies, earning her an average €150,000 a year.
According to Catalan newspaper La Vanguardia, she was sentenced in 2011 for the alleged rigging of several public entrance examinations for the council she presided over.
Her career was described as "intense" and "often controversial" by El Pais.