The Israeli navy was surrounding a ship attempting to break the sea blockade of Gaza on Saturday, passengers said.
The boat, which is carrying two olive trees, 41 tons of cement, books, toys and medical equipment, was flanked by Israeli navy ships, said Reut Mor, an Israeli activist.
It was about 30 nautical miles off the coast of Gaza, the activist said.
Those onboard the ship are hoping to call attention to the blockade of the Palestinian territory, which has been under an Israeli blockade since 2007.
Among those sailing are a priest, a lawmaker, along with Mor.
"I want to say that we are here to give a message of solidarity to the people of Gaza," Mor said from aboard the Estelle earlier.
"The siege is inhuman and immoral,
Vessels attempting to break Israel's blockade of Gaza have sparked controversy -- and violence -- in the past.
In 2010, an Israeli raid on one flotilla ship, the Mavi Marmara, resulted in the deaths of nine Turkish activists, deteriorating relations between Israel and Turkey, once close allies. Israel was roundly criticized by many over the deaths.
A U.N. report criticized Israel for its use of excessive force in the incident, but described the blockade -- which activists call illegal -- as a "legitimate security measure."
Israel says it is concerned about the smuggling of arms to Gaza militants intent on attacking the Jewish state. Gaza is controlled by the anti-Israel Hamas militant group, regarded as a terrorist group by the United States and Israel.
But activists say Israeli embargoes of goods into Gaza from land and sea are collective punishment of civilians in what is a tiny and densely populated strip of land along the Mediterranean coast.
Israel has said any organization or state that wants to give humanitarian aid to Gaza can do so in coordination with Israeli authorities via existing land crossings into the Palestinian territory.