US President Donald Trump has hit back at the judge who has overturned his travel ban, describing him as a "so-called judge".

As thousands of people in the UK marched in London to protest President Donald Trump's state visit to the UK and his travel ban, a judge in Seattle has succeeded in suspending the ban.

Judge James Robart ruled that the ban on visitors from seven Muslim countries entering the United States is unconstitutional.

Seattle attorney general, Bob Ferguson, said: "This decision shuts down the executive order right now. No one is above the law, not even the President." He was quoting Lord Denning in his confrontation with Attorney General Sam Silkin in 1977. Denning himself was himself quoting English churchman Thomas Fuller from three centuries earlier.

Trump tweeted: "The opinion of this so-called judge, which essentially takes law-enforcement away from our country, is ridiculous and will be overturned!"

The White House immediately condemned the ruling as airlines around the world and airports in the US ceased implementing it. Trump's administration has already launched an "emergency stay" against Robart's ruling.

Qatar Airways, Air France, Iberia and Lufthansa  were among the first airlines to start allowing the banned passengers to board again.

Air France spokesman Herve Erschler said: "Nationals from the countries concerned are being authorised to fly once again to the United States, providing their papers and visas are in order."

There is uncertainty still over how many banned travellers will reinstate their plans to visit the US

Marwa al-Naal, a US citizen working in Syria and married to a Syrian man eligible for residency in the US said they were still afraid to return.

"A lot of people did advise me to go travel yesterday back to Boston. I did not take that flight because there was still the risk of facing detention," she told the BBC.

Tariq Laham, 32, and his Polish fiancee Natalia, of Dubai, who had scrapped plans to travel to the United States after they get married in July in Poland, will alsonot reverse their decision.

"It is just too risky," said Laham, a Syrian who works as a director of commercial operations at a multinational technology company. "Every day you wake up and there is a new decision," he told Reuters. 

The challenge in Seattle was brought by the state of Washington and later joined by the state of Minnesota. The judge ruled that the states have legal standing to sue, which could help Democratic attorneys general take on Trump in court on issues beyond immigration.

Washington's case was based on claims that the state had suffered harm from the travel ban, for example students and faculty at state-funded universities being stranded overseas. Amazon.com and Expedia both based in Washington state, had supported the lawsuit, asserting that the travel restrictions harmed their businesses.

Tech companies, which rely on talent from around the world, have been increasingly outspoken in their opposition to the Trump administration's anti-immigrant policies.

Judge Robart probed a Justice Department lawyer on what he called the "litany of harms" suffered by Washington state's universities, and also questioned the use of the Sept. 11, 2001, attacks on the United States as a justification for the ban.

Robart said no attacks had been carried out on U.S. soil by individuals from the seven countries affected by the travel ban since that assault. For Trump's order to be constitutional, Robart said, it had to be "based in fact, as opposed to fiction."

In a statement, the White House insisted the ban was lawful: "The president's order is intended to protect the homeland and he has the constitutional authority and responsibility to protect the American people."

Additional reporting by Reuters