International rail operator Eurostar said it would cancel a quarter of its services on Friday and over the weekend, after France's high-speed rail network faced coordinated acts of vandalism just ahead of the opening of the Olympic Games in Paris.

Domestic rail services were also widely disrupted.

French railway operator SNCF said in a statement that its network had experienced several concurrent attacks early Friday morning, with fire damaging its facilities and its fiber optic lines. The high-speed LGV Atlantique line between Paris and Bordeaux was hit, along with northern and eastern lines, causing knock-on disruption that SNCF expected will impact hundreds of thousands of people through the weekend.

SNCF later said that a "very gradual resumption" of services had taken place since 1 p.m. local time in France, with delays still continuing across the country's high-speed rail network. The company is expected to give an update on the situation later this afternoon.

Eurostar and SNCF advised passengers to delay or cancel travel if possible, offering free journey changes or refunds.

The attacks come just ahead of the opening ceremony of the Olympic Games, due to take place along Paris' River Seine at 7:30 p.m. local time (1:30 p.m. ET) on Friday. The display is set to feature tens of thousands of athletes and spectators along with a heavy police presence.

Eurostar services, which connect the U.K., France, Belgium, the Netherlands and Germany, were hit by disruption to the high-speed line between Paris and Lille in northern France.

Passengers are being diverted onto local, slower lines to and from the French capital, extending journey times by around an hour and a half, alongside the cancellations.

Transport Minister Patrice Vergriete condemned what he described as "criminal acts" that were carried out simultaneously at several locations. Vergiete told a press briefing that an investigation is underway, according to NBC News reporting. 

In a briefing to French television, SNCF President Jean-Pierre Farandou said 800,000 people would be affected by the disruption across the weekend.

"We are so sorry to not be able to circulate the trains expected by the French. Today the major departure lines are attacked, an attack on the French," Farandou said, according to an NBC News translation.

"The celebration is spoiled."

It was not immediately clear who perpetrated the incident. Farandou said the attacks were carried out at 4 a.m. across several locations, with fiber optic lines affected and explosive material found on sites. Thousands of workers will be mobilized to reestablish the lines, he added.

French Prime Minister Gabriel Attal said on social platform X that he was grateful to firefighters who had responded to affected sites and to SNCF workers who would be restoring services.

Attal also said that he was thinking of all the French people and families who were preparing to go on vacation, and that he shared their anger. Parisians are known for departing the city en masse in August for their summer breaks, with numerous offices and hospitality venues shutting down.

Protesters and activists have spoken out against several issues surrounding the Games in recent weeks, flagging the environmental and social impact of the proceedings and the inclusion of Israeli athletes amid the ongoing war in the Gaza Strip.

French officials have been on high alert for disruption to the Games, with 45,000 police officers due to be deployed for the opening ceremony, according to Reuters. Tensions are particularly high due to the recent assassination attempt on former U.S. President Donald Trump, as well as Paris' history with deadly terror attacks.

Areas of the city center, including around the Louvre, Notre Dame and the Eiffel Tower, have been restricted to access only via a QR code or to workers.

"There has been a bit of complexity around the environment in terms of being able to secure around the Seine, so the traffic has been a bit complicated, walking around has been a bit complicated, but I'm sure we'll have an absolutely outstanding opening ceremony today and a great Olympics," Aiman Ezzat, CEO of French tech firm Capgemini, told CNBC's "Squawk Box Europe" early Friday.

Correction: Jean-Pierre Farandou is president of SNCF. An earlier version misspelled his name.

Disclosure: CNBC parent NBCUniversal owns NBC Sports and NBC Olympics. NBC Olympics is the U.S. broadcast rights holder to all Summer and Winter Games through 2032.