Spain's King Felipe (C) looks down as he stands along politicians including Prime Minister Mariano Rajoy (3rdL) and President of the Generalitat of Catalonia Carles Puigdemont (3rdR) while they observe a minute of silence in Placa de Catalunya, a day after a van crashed into pedestrians at Las Ramblas in Barcelona, Spain, Aug 18, 2017. [Photo/Agencies]

The King of Spain, the prime minister of Spain and the president of Catalonia joined thousands for a minute's silence at the Placa Catalunya, the point where the driver started his rampage that killed 13 and injured more than 100 on Thursday.

King Felipe, Mariano Rajoy and Carlos Puigedemont put on a display of unity as Spanish police continued to search for suspects involved in a series of deadly incidents on Thursday and Friday.

President Xi Jinping, and Prime Minister Li Keqiang have both written to King Felipe of Spain to express their condolences over the terror attack in Barcelona.

In his letter, Xi mourned the dead and expressed sympathy to the bereaved families and the injured, on behalf of China. "China is firmly opposed to all forms of terrorism and stands ready to work with the international community, including Spain, to strengthen anti-terror cooperation so as to jointly safeguard regional and world peace and stability," he wrote.

Spanish police said they had identified a cell of jihadists operating in the northern Spanish region of Catalonia, after at least four separate incidents which highlighted the difficulty of protecting the public from a driver with evil intentions.

The most serious incident occurred in Barcelona when a man drove a white van 500 meters down the city's most famous boulevard, Las Ramblas, on Thursday, killing at least 13 people and injuring more than 100.

The street was full of tourists and the victims were from all over the world. Three of the injured were Chinese; two from Taiwan were still being treated in hospital while a third from Hong Kong suffered minor injuries and was discharged from hospital.

Unlike in London, where bollards and road blocks have been placed between popular pedestrian areas and roads after two vehicle attacks, the driver was able to drive freely down the wide pedestrian zone at the centre of the Ramblas, veering to the left and right in search of victims.

Police arrested two people but are still searching for the driver, who they named as 18-year-old Moroccan Moussa Oukabir. Police also arrested his brother Driss Oukabir,28, who they said hired the van, in Ripoll near the border with France. Oukabir told police that he had not rented the van and that his identity papers had been stolen.

Later, in the early hours of Friday morning, an Audi A3 was driven at pedestrians and police in the coastal town of Cambrils, south of Barcelona, killing one person. The driver and four passengers were shot dead by police. Five bystanders and one police officer were injured. Police said that the men were wearing fake bomb vests.

Catalan emergency services say people of more than 30 different nationalities were?killed or injured in Barcelona and Cambrils terror attacks.

Two other incidents in Catalonia could also be linked to the incidents in Barcelona and Cambrils. In San Jus Deveran, outside Barcelona, police opened fire on a car at a road block. They discovered the car owner stabbed to death in the passenger seat but are still searching for the driver.

It also emerged that an explosion that happened earlier on Thursday in a house in the seaside town of Alcanar could have been caused by people trying to make bombs. One person was killed and one injured in the blast, which authorities first thought was a gas explosion.

Police are still trying to piece together the different incidents and how and if they are connected. What is clear is that the Barcelona attack highlights how vulnerable any city is to someone who wants to kill a lot of people by?using a vehicle and that police and secret service intelligence cannot stop every attack.

It is also clear that such attacks that are aimed at tourists from all over the world are more likely to unite people than divide them.

The Islamic State in Iraq and Syria claimed the attack but without giving any information that demonstrated they had any connection with the individuals involved. ISIS is believed to have inspired recent vehicle attacks in London, Stockholm, Berlin and Nice.

ISIS controlled-territory in Iraq and Syria is being re-taken forcing them to look for new areas of operation and to demonstrate their reach and power even as their strongholds are defeated.

But al-Qaeda linked groups remain strong in Syria and Yemen. Al-Qaeda in the Arabian Peninsula published the latest edition of its Inspire magazine in which they encourage supporters to attack the European rail network and derail trains.

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