Troublemakers sparked panic sparked at Seville's nighttime Good Friday processions, famed for their religious floats, hooded penitents and crowds of spectators, seriously injuring one person, Spanish authorities said.
Emergency services said eight people were detained in connection with the incidents, from 04:00 local time (0200GMT) on Friday, which sent people running in panic and leaving children in tears along procession routes throughout the city .
One man was later released, though he could still face charges, authorities in the southern Spanish city said.
In a statement, the Cecop centre that oversees security during the annual processions said those detained had "shouted", used metallic objects to make loud noise or made "wild gesticulations" to create panic in the thousands-strong crowds.
An AFP photographer present said she heard what sounded like a stampede of galloping animals, and then a mass of people pushed towards her.
Standing on the Isabel II bridge that straddles Seville's Guadalquivir River, she climbed onto a lamppost.
"There were children, women with prams," she said, adding that some people rushed down steps towards the river, falling over themselves in panic.
"The first thing people think is that there is a terrorist attack."
Spanish media reported that some of those later detained shouted "Allah is great" as well as slogans in favour of the Basque separatist group ETA.
But Ricardo Gil-Toresano, the central government's representative in Seville, said there was no link to extremism.
"It's got nothing to do with Islamist groups, ETA, or any known terrorist group," he told reporters in comments broadcast on Spanish television.
"They wanted to create the utmost panic with their shouting," he added.
An initial probe showed that there were three initial movements of panic, which sparked a "domino effect" in other parts of the city, Cecop said.
"These are isolated cases without any apparent connection that are similar to cases of vandalism and hooliganism," it said.
The situation was later brought back under control and the processions continued.
Cecop said three of those arrested were "common delinquents".
Some 17 people were taken to hospital for injuries and panic attacks, it said.
One of them was in intensive care in a serious condition with a head injury.
Organised by religious brotherhoods and featuring huge floats of wooden sculptures of religious scenes accompanied by hooded penitents, the processions known as "La Madruga" are the high point of Easter Week festivities in Seville.
This morning's early incident mirrors a similar outbreak of panic in 2000 in Seville's Good Friday processions, which left 52 people injured.
Good Friday is the second of four intensive days in the Christian calendar culminating in Easter Sunday, which commemorates Christ's resurrection.