The woman, who is eight months pregnant, was walking through the centre of the seaside city last week with her husband and two children, and was rebuked by the two ultras because she was wearing the Islamic veil, police said in a statement.
Her husband reacted and was assaulted by the pair, who have not been named but are said by police to have links to the far-right Brigadas Blanquiazules group, which supports the Espanyol football team in Barcelona.
The club banned members of this group from entering its stadium in 2010.
The woman tried to intervene and one of the ultras kicked her in the stomach, police said, adding the two were subsequently detained and have since been accused of hate crime, discrimination and personal injury.
Hospital checks found no damage to either the woman - who has not been named - or her baby, police said.
The assault comes with anti-Islam hate crimes on the rise in Spain.
In April, Mounir Benjelloun, head of the Spanish Federation of Islamic Religious Entities, told AFP 534 anti-Islam incidents were reported in Spain last year, a more than tenfold jump from 2014.
He said these acts of violence increased whenever there was a high-profile extremist incident elsewhere, such as the January 2015 attack against satirical weekly Charlie Hebdo, police and a kosher supermarket in Paris.
"We're talking about this case because it has come out into the open," Benjelloun told AFP Thursday.
"But there are incidents in Spain every day."
In March, people linked to a far-right group protested outside one of Madrid's main mosques after the deadly Brussels airport and metro attacks.
The protesters gathered at the Omar mosque and placed a large placard that read: "Today Brussels, tomorrow Madrid?".
Several mosques have also been vandalised.