GUN salutes sounded across London today to mark the birth of the royal baby as the armed forces join in the celebrations.
The bells rang from Westminster Abbey to herald the arrival of the new prince.
The King's Troop Royal Horse Artillery and the Honourable Artillery Company carried out the ceremonial royal salutes in honour of the new addition to the Royal Family.
Gun salutes are fired for the birth of every prince or princess, no matter where their place is in the line of succession, the Ministry of Defence said. The last royal salute for a birth was for Princess Eugenie in 1990.
The King's Troop Royal Horse Artillery, wearing full dress uniform, will ride past Buckingham Palace to Green Park where they will stage a 41-gun royal salute.
They will ride from their forward mounting base in Wellington Barracks into Green Park, where 71 horses will pull six First World War-era 13-pounder field guns into position for the royal salute at 2pm.
Each of the six guns will fire blank artillery rounds at 10-second intervals until 41 shots have been fired. The horses and riders will then collect the guns and escort them back to Wellington Barracks.
Major Mark Edward, commanding officer of the King's Troop Royal Horse Artillery, said: "The opportunity to mark the birth of the child of the Duke and Duchess of Cambridge by firing a 41-gun royal salute, comes as a huge honour for the King's Troop Royal Horse Artillery."
The Honourable Artillery Company (HAC), the City of London's Army Reserve Regiment, will fire a 62-gun salute from Gun Wharf at the Tower of London at 2pm.
Whilst a royal gun salute normally comprises 21 guns, this is increased to 41 if fired from a royal park or residence.
Uniquely, at the Tower of London, which is a royal residence, 62 rounds are fired as this also includes an additional 21 guns for the citizens of the City of London to show their loyalty to the monarch.
Celebrations are not confined to London, as all of Her Majesty's ships - in home or international waters - are flying the Royal Navy Ensign flag from their mast to mark the birth of the prince.
HMS Lancaster, deployed on a patrol task in the Caribbean, added its own congratulatory message to the Duke and Duchess, spelling out "It's A Boy!" on the deck of the ship.
Commanding officer Commander Steve Moorhouse said: "As 'the Queen's Frigate' we were particularly delighted to hear the fantastic news of the royal birth and everybody on board HMS Lancaster sends their warmest congratulations and best wishes to the Duke and Duchess of Cambridge."
Staff from RAF Valley, where Prince William serves as a search and rescue pilot, were among the first to offer their congratulations.
Station commander Group Captain Adrian Hill sent a congratulatory message to the couple on behalf of the RAF Valley.
"We're all delighted to learn that Flight Lieutenant Wales and the Duchess of Cambridge are now proud parents of a new-born son," he said.
"We wish them all the very best for the future. From everyone at RAF Valley, congratulations."
The Ministry of Defence said other units connected to WIlliam also marked the birth, including the Army regiment he was commissioned into from Sandhurst, the Household Cavalry, which is partly deployed in Afghanistan
The Irish Guards, of which William is the colonel, toasted the birth with a traditional glass of Irish stout at their barracks in Aldershot and at bases in London.
Kate visited the battalion this year and last year to present shamrocks to all ranks on St Patrick's Day, a role carried out for many years by a female member of the Royal Family.
Commanding officer Lieutenant Colonel Ed Boanas said: "On behalf of all ranks 1st Battalion Irish Guards, I send our warmest congratulations and best wishes to the Colonel of the Regiment and HRH the Duchess of Cambridge on the occasion of the birth of their son, with the heartfelt hope that he has a long and happy life. Quis Separabit (who will separate us?)."
Trafalgar Square's famous fountains were lit with blue lights last night and London mayor Boris Johnson said they will be lit for the next six days in celebration of the royal birth.
"Huge congratulations to the Duke and Duchess on the birth of their first child who will, by virtue of being born in this great city, be a Londoner through and through," he said.
"I am sure millions of Londoners will be raising a glass today to toast this wonderful news."
London's Living Room at the top of City Hall will also use blue lighting over the next six days to celebrate the royal birth.
Westminster City Council celebrated the announcement of the baby boy's birth with blue lighting in the fountains in Marble Arch and the Golden Jubilee Bridge. They will remain lit up until midnight tonight.
Council leader Philippa Roe said: "Just like everyone else around the country, we are obviously thrilled at the news. We also give a special welcome to our latest Westminster arrival and, of course, add our congratulations to the Duke and Duchess of Cambridge.
"We will be working with Kensington Palace to register the birth of the baby."
London's Natural History Museum, of which Kate is patron, is celebrating the birth of the new prince by lighting up its main Waterhouse building blue at sunset every night for three nights.
Michael Dixon, director of the museum, said: "Following the announcement earlier this year that the Duchess has chosen to be the patron for the Natural History Museum, we are overjoyed to receive news of the royal birth.
"I send the Duke and Duchess my best wishes and I am very much looking forward to welcoming another member of the royal family to the museum."