Norway attacks: Queen expresses UK's sympathy
The Queen has written to Norway's King Harald to express sympathy over the attacks in his country, saying her thoughts are with Norwegian people.
A bomb attack in Oslo left seven people dead before a gunman opened fire at a youth camp, killing at least 84 people.
David Cameron spoke to Norway's PM and the UK has offered police assistance.
In her message, the Queen said: "I am deeply saddened and shocked by the tragic loss of life of so many people on the island of Utoeya and in Oslo."
She added: "Prince Philip joins me in extending our heartfelt sympathy to your majesty and the people of Norway. Our prayers and thoughts are with everyone who has been affected by the dreadful atrocity."
The Norwegian embassy in Belgrave Square, central London, was open on Saturday, its flag flying at half mast.
A spokesman said: "We're open for any Norwegians who might need someone to speak to or feel the need to be with someone."
At least seven people were killed in the bombing and scores more are known to have died at the camp for young members of the Labour Party in Utoeya, an island outside the capital.
Police have charged a 32-year-old Norwegian man, Anders Behring Breivik, over both attacks.
The Foreign Office has advised British nationals in Norway to take extra care.
In a statement on its website, the Foreign Office said: "We recommend that British nationals stay indoors for the time being. British nationals are advised to exercise caution, monitor local media reporting and follow advice given by the emergency services."
Diplomats also say they are checking whether any British nationals are affected by the tragedy.
Around 250,000 British tourists visit Norway every year, the Foreign Office says on the Norway travel advice section of its website.
And according to the UK embassy in Oslo, there are also 25,000 to 30,000 Britons resident in the country.
Prime Minister David Cameron has said he was "outraged" by the attack that had led to an "absolutely horrific" loss of life.
Mr Cameron said: "It's on a scale, frankly, that is hard to comprehend. The Norwegians are old friends and allies and neighbours of Britain, and I know that everyone in Britain will want to stand with the Norwegian people in the days of sorrow that lie ahead.
"Also, we'll want to make sure that we learn like others, any lessons there are to learn about how to be more secure against horrific outrages like this. And that's something we can discuss at the National Security Council on Monday."
'Ready to assist'
He also said he had spoken to the country's PM Jens Stoltenberg to express the UK's condolences and offer assistance in tracking down the perpetrators.
Home Secretary Theresa May said she had also spoken to Norwegian Justice Minister Knut Storberget on Saturday morning.
"I told him that the Norwegian people remain foremost in our thoughts and that the loss of so many young people is particularly tragic.
"I made clear to Minister Storberget that we will help in any way we can. In particular I offered police assistance, which we stand ready to provide should Norway request it," she said.
Meanwhile, First Minister Alex Salmond expressed condolences on behalf of Scotland and said his government had offered help.
"Scotland and Norway enjoy very strong bonds of friendship stretching back over many years, and our thoughts and sympathies are today with the Norwegian people, particularly the families of all those affected by these terrible incidents," he said.
The Mayor of London, Boris Johnson, has written to the mayor of Oslo, saying: "Our thoughts are with those who have died, the bereaved and those injured."