A spectacular boat procession down the Seine at the opening ceremony of the Olympic Games could be used by national delegations seeking to show “solidarity” with the Palestinian cause.

Around 90 vessels carrying 10,500 athletes and officials are due to float in convoy through Paris on Friday night, including a delegation from Israel. Pro-Palestine organisers said discussions were taking place about a high-profile demonstration.

A “show of solidarity” could involve some unnamed national delegations flying the Palestinian flag or wearing messages on clothing, sources said, as part of a policy of those opposed to Israel’s involvement in the Games to target open-air events over the next three weeks.

A spokesman for Team Israel said that none of their athletes had opted out of taking part in the convoy down the river but he declined to comment on the number of individuals that will be boarding for the 6km (3.7 miles) trip.

Stephanie Adam, a campaigner with the Palestinian Campaign for the Academic and Cultural Boycott of Israel, said: “While the International Olympic Committee and host country France would like to hide the dissent, including through repressive and underhanded tactics, the strong popular opposition to genocidal Israel’s presence in the Olympics made itself heard and seen at the Israel-Mali Olympic football match on Wednesday. The dissent will only grow during the Games.

“We call on participating delegations to join calls for a ceasefire now and to stop Israel’s Gaza genocide. We urge Olympic athletes to use their platform to take a stand for Palestinian rights.”

Security will be tight at the opening ceremony, with 45,000 police and gendarme being supplemented by a further 2,000 security guards, according to Bruno Le Ray, the Games’ director of security.

Le Ray said he was satisfied with the first few days of events at the Games, saying: “The first lesson we can draw from this is that in terms of safety and security we have had no incident on any site, on any of the football matches, despite the fact that some of the meetings have been deemed to be slightly more sensitive, particularly the match between Israel and Mali.”

When questioned about a pitch invasion in Saint-Étienne that led to a match between Argentina and Morocco being suspended for more than an hour before resuming behind closed doors, Thomas Collomb, the deputy director of security, said the crowd had believed that the game was over. He said: “The event of yesterday was a security instance but not a major incident. Five hundred people were on site to secure the game … but following the last goal, when the referee blew the whistle, people thought it was the end of the match. They flooded the pitch because they were happy because they thought it was the end of the match. As far as we are concerned the security link was very low. There was no major security incident at all throughout this event.”

A spate of violent threats have been made against Israeli athletes ahead of the Games and on Thursday Israel’s foreign minister, Israel Katz, sent a letter to his French counterpart, Stéphane Séjourné, with a fresh warning of an Iranian-backed plot to attack the Israeli delegation.

“There are those who seek to undermine the celebratory nature of this joyous event,” Katz wrote. “We currently have assessments regarding the potential threat posed by Iranian terrorist proxies and other terrorist organisations who aim to carry out attacks against members of the Israeli delegation and Israeli tourists during the Olympics.”

Katz expressed “gratitude” to French officials for the “unprecedented security measures” to protect Israelis at the Games, as well as the French government’s rejection of calls to ban Israel from the Olympics.

The 88 Israeli athletes at the Games are subject to round-the-clock protection from the French security services, and are also being guarded by officials from the Shin Bet intelligence agency.

Meanwhile, police in Belgium arrested seven people on Thursday on suspicion of preparing a “terrorist attack”, following 14 raids nationwide. A spokesman for the federal prosecutors’ service said there was no evidence of any link to the Paris Olympics.

There are “no details at this time as to the locations or targets but what was found leads us to believe an attack was being prepared”, the spokesman said.