By Agence France-Presse
The gang of five men armed with hatchets smashed display windows on the ground floor of the hotel in Paris' luxurious Place Vendome around 6:30 pm (1730 GMT), making off with the jewels.
Three of the gang were detained while fleeing the scene and an AFP journalist said some roads around the hotel were sealed off by security forces following the incident. Two people were still being sought by authorities on Thursday evening.
"We heard a loud noise and lots of racket in the street," one hotel employee told AFP. "Passers-by took refuge in the hotel. We didn't know what was going on until someone told us there had been a robbery."
Another employee said he saw a motorbike speed along a road at the back of the hotel after the break-in.
Several luxury brands display their jewels in the street-front windows of the Ritz.
A judicial source put the value of the jewels seized at "more than four million euros" ($4.75 million).
Interior minister Gerard Collomb praised officers' "professionalism", saying they had "done our police force credit".
Paris prosecutors have opened a probe into the robbery, according to a judicial source.
Riches and robbers
Place Vendome, with its opulent window displays of jewellers and high-end watchmakers, has been the scene of several audacious daytime raids.
Security had been boosted in the area, which also houses France's justice ministry, in 2014 after several daring armed raids on Place Vendome in which jewels worth between 420,000 and two million euros were seized.
But thieves struck again in March 2016, when an estimated six million euros worth of jewels were stolen from luxury fashion brand Chopard by robbers who threatened employees with a gun and grenade. Three men were charged in connection with that heist.
The French capital's most high-profile recent jewellery theft was carried out in October 2016 against US reality television star Kim Kardashian.
Five men, some wearing jackets with police insignia, held Kardashian at gunpoint, making off with several pieces of gold and diamond jewellery as well as a ring -- a total estimated worth of nine million euros.
One of the robbers, fleeing the scene on a bicycle, dropped a diamond-encrusted cross worth 30,000 euros, which was found by a passer-by a few hours later.
It remains the only piece to be recovered from the heist.
The landmark Paris Ritz hotel re-opened its palatial doors to guests in 2016 after nearly four years of renovations and a major fire.
The hotel founded by Swiss entrepreneur Cesar Ritz in 1898 is storied as the lodging of choice of Charlie Chaplin, Coco Chanel and Ernest Hemingway, who has a small bar named after him.
In more recent history however, the Ritz has become known as the place where Britain's Princess Diana spent her last hours before a car accident in a tunnel in the French capital while being pursued by paparazzi.