King Abdullah of Saudi Arabia has been welcomed ceremonially to Britain by the Queen, amid burgeoning controversy over the Middle Eastern ruler's stay.
The Queen greeted him on Horseguard's Parade at the start of his first visit to the UK in 20 years.
The Lib Dems and some charities said the visit should not take place because of the kingdom's human rights record.
King Abdullah's remarks about the 7 July terror attacks have heightened the controversy surrounding the trip.
The Foreign Office was forced to rebut his claims that Saudi authorities had provided information which could have prevented the London bombing atrocities.
Convoy of limousines
Prime Minister Gordon Brown, Home Secretary Jacqui Smith and Minister for the Middle East Kim Howells joined dignitaries to welcome the king as he arrived at the central London parade ground with the Prince of Wales.
The head of the Metropolitan Police, Sir Ian Blair, who led the force through the London terror attacks of 2005, also greeted the king with a handshake and a smile.
The king reviewed a guard of honour before heading to Buckingham Palace in carriages.
After lunch with the Queen and the Duke of Edinburgh, the king was shown a specially created exhibition of Saudi items from the Royal Collection.
He examined a large 15th century Koran, given to Queen Victoria in 1898, and a diamond-encrusted gold sword presented to King George VI by King Ibn Saud in 1943.
The Queen also showed King Abdullah pictures of one of her racing horses, Banknote, which is descended from a Saudi gift horse.
On Tuesday evening, the Queen will host a state banquet attended by more than 170 people, in which both she and the king will make speeches.
The prime minister will be there, accompanied by wife Sarah, and dressed in white tie and tails.
Mr Brown declined to wear the official uniform of coat tails and white bow tie at the Mansion House dinner in June, but for this event his spokesman said he would "abide by the dress code".
BBC security correspondent Frank Gardner said six planes brought the king and his huge entourage to Heathrow airport.
It took three hours for all the luggage to be unloaded while a convoy of 84 limousines drove the party into London, our correspondent said.
Among them were 23 personal advisors who are staying at Buckingham Palace and more than 400 aides who are spilling out into London hotels, he added.
Around 100 human rights and anti-arms trade activists jeered and shouted "shame on you" as the royal procession passed along The Mall in central London.
Human rights activist Peter Tatchell said it was "incredible hypocrisy" for ministers to condemn the Burmese and Zimbabwean regimes while saying nothing about human rights abuses in Saudi Arabia.
He said: "It just shows oil and arms sales seem to have bought the government's silence. "
On Monday, Foreign Office minister Kim Howells called for Britain and Saudi Arabia to work more closely together, despite their differences.
He said the two states could unite around their "shared values".
Acting Liberal Democrat leader Vince Cable said he was boycotting the state visit, while backbench Labour MPs are planning to join human right demonstrators at a protest outside the Saudi embassy on Wednesday.
Downing Street has insisted the government would continue to raise the issue with the Saudi delegation during the week, but said it would not "dominate" talks.
During his visit, the king will also meet Prince Charles to discuss the Prince's Trust and will attend a banquet hosted by the Lord Mayor at the Guildhall, before returning home on Thursday.