"Unfortunately we were stopped and they came on baord about 10 miles from the 25 miles line. We didnt resist and gave full co-operation," the activist said.
Meanwhile, Jacky Rowland, Al Jazeera's correspondent in Ashdod, said the vessel Rachel Corrie is being held at the port while its cargo is inspected.
"The aid will be taken off and inspected," she said. "It is not clear how much of it will be taken to Gaza."
The seizure of the Rachel Corrie came five days after Israeli troops killed nine activists while raiding a flotilla of aid ships carrying humanitarian supplies for the beseiged Palestinian territory.
Israeli naval vessels trailed Rachel Corrie for several hours before boarding it, jamming its radar and cutting off all radio communication.
Binyamin Netanyahu, the Israeli prime minister, said naval forces involved in Saturday's operation had followed exactly the same procedures as they had done earlier in the week, but the difference lay in the attitude of the activists on board the vessel.
"We saw today the difference between a ship of peace activists, with whom we don't agree but respect their right to a different opinion from ours, and between a ship of hate organised by violent Turkish terror extremists ... waiting for our soldiers on the deck with axes and knives," Netanyahu's office cited him as saying.
Mukhriz Mahathir, head of the Perdana Global Peace Organisation, a Malaysian non-governmental organisation that sponsored the Rachel Corrie, said people would be willing to contribute to further aid ships.
Al Jazeera reports from London on protests against the Israeli raid and embargo on Gaza
"These actions have finally succeeded in raising lots of awareness," he said.
"But for the moment, we are upset that these aid goods have not reached their destination."
The Rachel Corrie is named after an American womanwho was killed by an Israeli military bulldozer in the Gaza Strip in 2003, while protesting against a house demolition.
Pro-Palestinian activists on board the ship had earlier said they were determined to press ahead towards the Gaza coast, despite the Israeli warnings.
"We are not afraid and we are all advocating non-violence… and we will just sit here and go if they insist on commandeering our boat and forcing us into Ashdod," Mairead Maguire, an activist and Nobel Peace laureate aboard Rachel Corrie, said before radio contact with the ship was lost.
The Israeli military had cautioned that troops would consider storming the ship, if it continued its journey towards Gaza.
Al Jazeera's Nicole Johnston, reporting from Gaza, said the Rachel Corrie had hundreds of tonnes of aid, including medical supplies, wheelchairs, cement, building materials and even note pads for children.
Speaking to Al Jazeera on Saturday, Chris Gunness, a spokesman for UNWRA, the United Nations Palestine refugee agency, underscored the need for Gaza's land and sea blockades to be lifted.
Israel's raid has led to protests in many nations including in the French capital Paris
"The World Health Organisation needs $20m worth of aid to enter Gaza, there is 44 per cent unemployment and education is in crisis," he said.
"We are turning thousands of five and six year olds away from our schools [and] 39,000 children in Gaza are not receiving United Nations education because we cannot get our building materials in.
"We also want to see exports coming out of Gaza. It is already 80 per cent aid dependent. What we don't want is to make it more aid dependent."
The developments surrounding the Rachel Corrie came as the UN human-rights chief said that Israel could face prosecution for Monday's flotilla raid, and that she was following requests for a referral to the International Criminal Court [ICC] in The Hague.
"I am following very closely the very many calls that come particularly from civil society and from all the people who are suffering in Palestine for that kind of action to be taken," Navi Pillay said on Saturday in Kampala, Uganda, where she was attending the ICC's ongoing review conference.
She said she believed Israel's blockade of Gaza violates international law.
"International humanitarian law prohibits starvation of civilians as a method of warfare ... It is also prohibited to impose collective punishment on the civilian population, so it is [for those reasons] that I have consistently reported to member states that the blockade is illegal and must be lifted," Pillay said.
"Even if it is demonstrated that the blockade is legal under international law, Israel's current military operations against the flotilla must be analysed from the perspective of its obligation to allow humanitarian aid to be brought into the Gaza Strip."