"Revolting", "cowardly", and "barbaric" are some of the words leaders worldwide have used to describe the attack in Barcelona that killed 13 people.

At least 100 others were wounded when a white van ploughed into a crowd in one of the Spanish city's busiest tourist districts.

The attack, which was claimed by the Islamic State of Iraq and the Levant (ISIL, also known as ISIS), occurred in the bustling Las Ramblas area, a 1.2km stretch of shops and restaurants usually heaving with tourists.

There were at least 18 nationalities among the victims, who came from countries as varied as France, Venezuela, Australia, Ireland, Peru, Algeria, Belgium and China, according to Spain's civil protection agency.

Immediately following Thursday's attack, leaders worldwide, as with as thousands of people online, condemned the incident. Here is some reaction:

Spanish Prime Minister Mariano Rajoy called the killings a "savage terrorist attack" and said Spaniards "are not just united in mourning, but especially in the firm determination to beat those who want to rob us of our values and way of life". He announced three days of mourning.

French President Emmanuel Macron expressed France's solidarity after what he termed a "tragic attack".

In Germany, Chancellor Angela Merkel described the incident as a "revolting attack", according to a statement by her office. Merkel's spokesman Steffen Seiber tweeted that Germany was "thinking with profound sadness of the victims ... [and] with solidarity and friendship alongside the Spanish people".

British Prime Minister Theresa May called the attack "terrible" and said the UK "stands with Spain against terror". London Mayor Sadiq Khan said: "London stands with Barcelona against the evil of terrorism."

My thoughts are with the victims of this barbaric terrorist attack in the great city of Barcelona and with their brave emergency services. — Sadiq Khan (@SadiqKhan) August 17, 2017

Dutch Prime Minister Mark Rutte condemned what he called the "cowardly attack against innocent people", adding that Thursday is a "black day at a place where so many people around the world gather".

In Belgium, Foreign Minister Didier Reynders sent his condolences to the family of the Belgian national who was among the victims. Prime Minister Charles Michel added on Twitter that "no barbaric act will undermine the power and resilience of our ally".

European Commission President Jean-Claude Juncker expressed his "profound sorrow and anguish".

In Australia, Prime Minister Malcolm Turnbull condemned the attack saying on Twitter his country is "resolute with Spain in the fight to defeat terrorism" and "love and prayers are with the victims".

Resolute with Spain in the fight to defeat terrorism we condemn the terrorist attack in Barcelona. Our love & prayers are with the victims. — Malcolm Turnbull (@TurnbullMalcolm) August 17, 2017

US President Donald Trump also condemned the attack on Twitter, saying the US "will do whatever is necessary to help". Secretary of State Rex Tillerson offered US assistance to Spanish authorities.

Russian President Vladimir Putin called for the world to unite in an "uncompromising battle against the forces of terror".

"What happened once again confirms the need for a genuine unification of efforts by the entire world community in an uncompromising battle with the forces of terror," Putin said in a telegram of condolences to Spanish King Felipe VI, describing the incident as a "cruel and cynical crime".

Pope Francis expressed "great concern", according to a statement from his office, which added he is praying for the victims.

#Barcelona

Online, #Barcelona trended worldwide with many expressing their solidarity with the city.

Still can't believe that something like this happened on my country..Praying for everyone involved. 🇪🇸❤️ #Barcelona #PrayforBarcelona pic.twitter.com/Rf86uReQts — Olivia Culpo Spain ♡ (@OliviaCulpoSP) August 17, 2017

All my thoughts and prayers are with the people of #Barcelona pic.twitter.com/GLMI0Zti8k — Mistborn (@Esi_Paintsil257) August 18, 2017

The Barcelona football club said its thoughts were with the victims with many of the footballers also tweeting their condolences.

Deeply sad about the attacks in Barcelona. My thoughts are with all the victims, their families and all the people of Barcelona. pic.twitter.com/mSUbzM5e5m — Jasper Cillessen (@JasperCillessen) August 17, 2017

Very shocked for what happened in Barcelona. All my support to the city and families! pic.twitter.com/YDtyAiw7Qc — Luis Suarez (@LuisSuarez9) August 17, 2017

In Paris, the Eiffel Tower's lights were turned off to remember the victims.

In New York City, the One World Trade Center building was lit up with the colours of the Spanish flag to honour those who died in Barcelona.

Tonight we lit the spire of @OneWTC red and yellow in solidarity with the people of Spain. #Barcelona pic.twitter.com/DEVpXQFhUW

— Andrew Cuomo (@NYGovCuomo) August 18, 2017

Officials have said there is "clearly" a link between the Barcelona attack and an explosion that took place on Wednesday in the town of Alcanar, which killed one person.

Spanish police said early on Friday they killed the alleged perpetrators of a suspected "terrorist attack" in the town of Cambrils, which authorities said "could be linked" to the incident in Barcelona.