THE HAGUE: The Dutch city of Utrecht where the Tour de France starts on Saturday is officially in the grip of a sweltering heatwave, Dutch meteorologists said, urging spectators to take proper precautions.

Temperatures for a fifth day soared past the 30 degree Celsius (86 degrees Fahrenheit) mark, "making the heatwave official," Royal Dutch Meteorology Institute (KNMI) chief spokesman Cees Molenaars told AFP.

An official heatwave in the normally temperate Netherlands requires five consecutive days with a temperature of 25 degrees Celsius or above, with at least three of those days recording a temperature of over 30 degrees.

Molenaars said the KNMI was particularly concerned about the hundreds-of-thousands of Tour spectators expected to stream to the central city of Utrecht and urged people to take necessary precautions including drinking plenty of water.

Forecasters predicted a maximum temperature of 32 degrees in Utrecht on Saturday, which could be exacerbated by the large crowds and the lack of a breeze within Utrecht's built up streets.

There is a possibility of thundershowers later in the afternoon to bring some relief, Molenaars said.

Meanwhile, thousands of spectators have already arrived in Utrecht, Dutch media reports said.

Many were well-armed against the sun with cooler boxes and bags well-stocked with beer and water.

Some spectators even tweeted pictures of setting up inflatable swimming pools next to the route for the Tour's Stage 1 individual time trial.