Human rights groups are investigating claims that Spanish police used excessive force to crack down on participants in Catalonia's referendum on secession.

At least 893 civilians and 431 police officers were injured in Sunday's clashes, according to regional and national authorities.

Human Rights Watch, a New York-based NGO, told Al Jazeera on Tuesday that it has sent a representative to Barcelona, the capital of Catalonia, to examine allegations it has received of police brutality.

"There are some serious allegations of excessive use of force by police against people who were assembling peacefully to express their views on the referendum," said Kartik Raj, a researcher with HRW.

Videos filmed on voting day appear to show Spanish security forces using a heavy-handed approach as they mobilised.

Some officers are shown wielding truncheons as others shot rubber bullets.

Reports said force was used under orders from Madrid to shut down polling stations and seize ballot boxes.

"The pictures and videos we have received so far indicate instances in which the police used force where they weren't facing imminent danger or physical threat," Raj said, speaking from Barcelona. "It appears there are cases that could have been handled altogether different and more peacefully. Our job, along with others, is now to investigate and verify.|

More than two million people, or 90 percent of those who cast a ballot, voted "Yes" to Catalonia breaking away from Spain.

Several of the more than five million people who were eligible to vote said they were prevented from casting their ballots.

Others who objected to Catalan secession abstained.

In June, about 40 percent of voters said they would vote "Yes" to independence, according to the Centre for Opinion Studies, the Catalan government's polling body.

Spain defends police action

READ MORE: 'Yes' wins Catalonia independence vote marred by chaos

The national government has defended Sunday's police action as proportional, with Spanish Prime Minister Mariano Rajoy thanking officials on Monday for acting with "firmness and serenity" in upholding the rule of law.

Rajoy had pledged to do everything in his power to prevent the October 1 vote from taking place following a judicial ruling last month - a day after the plebiscite was announced - by the country's constitutional court ordering its suspension.

Spain's 1978 constitution decrees that the country is indivisible, and grants the national government exclusive power to hold referendums.

Witnesses of Sunday's referendum, however, said police had acted indiscriminately in targeting both those who supported and opposed independence.

"The police didn't beat just people who were going to vote 'Yes', they forced and kicked at everybody, old people included," Pau Subira Zurita, 22, told Al Jazeera from Barcelona.

READ MORE: Catalonia referendum - What happens next?

Human rights observers on the ground on Sunday confirmed there had been a number of incidents of unwarranted police aggression, according to IRIDIA, a Catalonian centre for the defence of human rights.

Anais Franquesa Griso, a human rights lawyer with IRIDIA, said the centre's 70 observers on Sunday reported several instances of the excessive use of force by security forces.

"Police officers were not acting in self-defence, we are completely sure of that," she told Al Jazeera.

"We have reports of peaceful protesters, including children and very elderly people, being injured."

The centre is working with other organisations, including HRW, to collect testimony from those who claim they were injured or had their rights infringed upon, on Sunday and in the period leading up to the referendum from September 20 onwards.

"We will be collecting information until October 7, and will then send a report to a number of international human rights organisations to inform them of what has been happening here," Griso said.

"It's a critical moment, and [Sunday] was a critical attack on democracy and human rights."

Meanwhile, Zeid Ra'ad Al Hussein, the UN's senior human rights official, on Monday said he was "very disturbed" by Sunday's violent scenes.

"With hundreds of people reported injured, I urge the Spanish authorities to ensure thorough, independent and impartial investigation into all acts of violence," he said in a statement.

The concerns over human rights came as a large number of Catalans and regional institutions observed a general strike on Tuesday in condemnation of the alleged police violence. The strike had been called for by Catalan trade unions.