Pro-Palestinian activists vowed on Wednesday to continue to defy an Israeli navy blockade around Gaza, a day after Israel seized their boat and detained 21 people taking aid to the territory.

Israel boarded a small ferry carrying activists to Gaza on Tuesday, intercepting the vessel in Gaza's Israeli-controlled coastal waters. Those detained included an Irish peace laureate, a former member of the U.S. Congress and charity workers from Bahrain.

The activists, called the Free Gaza Movement, first started sending aid directly into Gaza in August 2008, but had been intercepted by the Israeli navy on two previous occasions.

The group would continue regardless of interceptions, said spokeswoman Greta Berlin.

"We are definitely going to go (back) even if we have to paddle across," she told journalists in Cyprus.

The activists had set off from Cyprus with humanitarian aid for Palestinians in Gaza, a territory ruled by Hamas Islamists. Israel tightened controls over Gaza, a sliver of territory of 1.5 million people, after Hamas took control there in 2007.

International calls for an end to the blockade have mounted after a 22 day Israeli offensive last December and January deepened hardship in the territory. Israel launched the military operation with the declared aim of ending cross-border rocket attacks.

Activists said they would continue to try to get aid directly to Gaza for as long as Israeli restrictions were in place. Israel says it is allowing aid to get through.

The International Committee of the Red Cross this week said stringent restrictions imposed by Israel are crippling reconstruction efforts.

"It is absolutely appalling how our small ships and a rag-tag band of activists has become the international conscience of what is happening there," said activist Ramzi Kyzia. "We are going to go, again and again."