Labour leader Jeremy Corbyn has branded the police crackdown against Catalonia's unofficial independence vote as "shocking".
Hundreds of people have been injured, some seriously, as the Spanish National Police and Civil Guard moved to shut down the vote, a Catalan government spokesman said.
Mr Corbyn tweeted: "Police violence against citizens in #Catalonia is shocking. The Spanish government must act to end it now."
Liberal Democrat leader Sir Vince Cable insisted Foreign Secretary Boris Johnson should call in the Spanish ambassador and tell him that the police response was "unacceptable".
Sir Vince said: "Police in a democracy should never drag people violently out of polling stations, whatever the arguments for or against holding a referendum.
"The police response looks to have been brutal and completely disproportionate.
"The Foreign Secretary should break off from conspiring against the Prime Minister and call in the Spanish ambassador to tell him that this is completely unacceptable."
The Spanish government and its security forces are trying to prevent voting in the independence referendum, which is backed by Catalan regional authorities.
Security forces have used rubber bullets and baton charges as they clashed with protesters, leaving some 460 people injured, according to Catalan authorities.
Scottish First Minister Nicola Sturgeon called on the Spanish government to "change course" and let people "vote peacefully" in the Catalan poll.
Ms Sturgeon expressed her concerns on Twitter, stating: "Some of the scenes in #Catalonia this morning are quite shocking and surely unnecessary. Just let people vote."
She added: "Increasingly concerned by images from #Catalonia.
"Regardless of views on independence, we should all condemn the scenes being witnessed and call on Spain to change course before someone is seriously hurt. Let people vote peacefully."
Scottish Conservative leader Ruth Davidson said: "Everyone will be shocked by the disturbing scenes coming from Catalonia.
"It is clear that this is a fast-moving situation, but we would urge the authorities to exercise restraint.
"Nobody wants to see people hurt."
Ms Davidson added: "If the situation in Catalonia is to be resolved, the answer will come through dialogue and diplomacy, and not through violence."