By Johan Ahlander and Mansoor Yosufzai

STOCKHOLM (Reuters) - Hours after the truck attack that killed four people in the heart of Stockholm, Muslim taxi driver Abdi Dahir found himself in a suffocating choke-hold from a man sitting in the back seat.

Struggling to breathe, Dahir, who moved to Sweden from Somalia as a child, felt he could die too, at the hands of an angry passenger who blamed the country's openness to Muslim immigrants for the attack that afternoon.

"We have done everything for everyone, we have given them mosques, we have given them everything but they kill our own people. Then we'll kill them," the man growled at Dahir before grabbing him around the neck, according to an audio recording of the assault.

Dahir, who recorded part of the conversation on his cell phone, said he had also activated a concealed alarm, prompting police to intervene. Stockholm police said they were investigating the incident.

"I tried to work, but I'm too nervous to have anyone sitting behind me in the car," said Dahir, his voice still hoarse.

Anti-Muslim anger is putting the Nordic country's deep-rooted liberal traditions to the test, after a man hijacked a beer truck and rammed it into a busy downtown pedestrian mall.

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At the time of the attack, the suspect, 39-year-old Rakhmat Akilov from Uzbekistan, had applied for asylum but had been rejected and faced an expulsion order, making him one of more than 12,000 people wanted for deportation in Sweden. In court on Tuesday his lawyer said he had confessed to a terrorist crime.

Europe's most welcoming nation to asylum seekers has tightened immigration policy in recent years and is considering new measures after Friday's attack, including better policing of deportation orders and banning membership of terrorist groups.

It is also bracing for rising intolerance and hate crimes.

"I'm quite worried about the political climate," said Mohamed Nuur, 25, a local politician from the ruling Social Democrat party who represents constituents in Rinkeby, a sprawling area of apartment blocks largely planned and built for workers in Sweden's progressive heyday of the late 1960s.

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It is part of a belt of heavily immigrant neighborhoods that ring Stockholm. Drab grey terrace houses line the streets where small shops advertise halal products in both Arabic and Swedish. Many women there wear head scarves when they go out.

"We already see manipulated images spread by Nazis and others who want to spread hateful messages," said Nuur, a Muslim of Somali decent.

Rinkeby has seen trouble such as riots among disaffected youths, but it looks well kept compared to many of the more run-down areas around major cities in Europe.

In Rinkeby metro station, Refa Jafari, a slight 23-year-old in a black baseball cap who came to Sweden as an unaccompanied minor from Afghanistan in 2010, stood handing out free SIM cards for a mobile operator.

"I will try to prove to Swedes that all people who have black hair are not Muslim and that all Muslims are not terrorists. We have been suffering the very same problem in our own countries," he said. "That is why we are here."

WHITE POWER

Fearing reprisals, Sweden's Security Police are stepping up surveillance of white supremacist groups.

"There is talk of taking revenge, of using violence, or a threat of violence, to show that we are angry about what has happened here," security police chief Anders Thornberg told local TV. Suspected right-wing extremists have planted bombs at asylum centers in the past, he noted.

Sweden, a country of 10 million people, has received around 700,000 asylum seekers since the end of the 1990s. In 2015, when hundreds of thousands of migrants entered the EU across the Balkans, Sweden took in a record 163,000, more than any other European Union member relative to the size of its population.

That huge migration wave from two years ago has since subsided after the EU reached a deal with Turkey to take back refugees and other migrants that cross illegally. But it caused soul-searching in Sweden, where the government tightened residency rules, cut benefits and imposed more border controls.

Support has grown for the anti-immigration Sweden Democrats, which opinion polls rank as the second biggest party behind Prime Minister Stefan Lofven's Social Democrats.

"We've not been taken seriously, maybe even a bit ridiculed, and I hope we can leave that behind us now," Sweden Democrats Vice Chairman Julia Kronlid told Reuters after the attack.

By far the biggest act of terrorism in the Nordic region was carried out not by an immigrant but by an anti-immigration fanatic, Anders Breivik, who killed 77 people in 2011 in an Oslo bomb attack and shooting rampage at a Norwegian summer camp.

Although hostility to immigration is still a political taboo for most mainstream political parties in the region, calls to shut the door are growing louder.

An hour after last week's attack, Christian Tybring-Gjedde, a lawmaker from Norway's ruling populist Progress Party, accused Lofven in a Facebook post of "implementing Europe's most irresponsible, naive and culturally self-destructive immigration policy".

Back in February, U.S. president Donald Trump attacked Sweden's immigration record, tweeting: "Give the public a break - The FAKE NEWS media is trying to say that large scale immigration in Sweden is working out just beautifully. NOT!".

Last week, before the truck attack, members of a vigilante anti-immigration group, Soldiers of Odin, which has expanded from Finland to the Nordics and Baltics, entered the gates of Al-Azhar, a Muslim school in Vallingby, a suburb near Rinkeby.

Assistant school head Roger Lindquist told Reuters two men had walked around the school yard days before the attack, taking photos of students and putting up stickers with their emblem. The school had felt compelled to hire security guards, he said.

"Why do they hate us? Why do they want to harm us and why do they want to shut down our school?" he said.

Sweden's liberal tradition is far from defeated, however.

Friday's truck attack prompted countless acts of generosity and solidarity and brought thousands of people onto the streets of central Stockholm in a show of unity against violence and extremism on Sunday.

"It was fantastic. A sea of people that came together and showed love, all kinds of people. That is what Sweden is about, and that is how we must keep it," said Juma Lomani, 32, who came to Sweden as a 19-year-old from Afghanistan and now works for an organization helping young refugees adapt to Swedish society.

(Additional reporting by Niklas Pollard, Johannes Hellstrom, Simon Johnson, Philip O'Connor and Johan Sennero in Stockholm, Stine Jacobsen and Teis Jensen in Copenhagen, and Terje Solsvik in Oslo; Writing by Niklas Pollard; Editing by Mark Bendeich and Peter Graff)