French and US activists hoping to defy Israel and breach its sea blockade on Gaza rallied outside the US embassy in Athens on Friday to demand their aid flotilla be allowed to sail from Greece.

Around 100 pro-Palestinians blocked traffic outside the embassy, waving peace flags and shouting slogans against French, Greek and US authorities which they claim are bowing to pressure from Israel to block the 10-boat mission.

Activists of all ages -- including firefighters, artists and members of parliament -- chanted "Gaza we are coming!" and "Free, free Palestine", as Greek police forcibly pushed them back with riot shields.

"We were driven back by the police... we're putting pressure on all the authorities who are trying to delay the flotilla's departure," said activist and politician Olivier Besancenot, of France's New Anticapitalist Party.

A handful of Greek would-be passengers who joined the rally said frustration was rife among the 300 or so activists from 22 countries who are hoping to carry medicines, a fully-equipped ambulance car and cement to Gaza.

The activists have blamed Israel's secret service for "sabotage" attacks carried out on two of the boats this week, and said Greece was complying with the Jewish state by inventing "bureaucratic excuses" to keep them grounded.

"Despite all the obstacles, all the attacks against us, we will win and we will leave for Gaza together," said Ann Wright, an organiser for the US Audacity of Freedom boat, proudly waving her stars and stripes flag.

Passenger Johnny Barber said it was "pretty clear the US embassy is not going to help", adding: "It's obvious they're complicit in stopping our boats."

Volunteers said they were prepared to set sail "whether we have official clearance to do so or not". Organisers have said they will wait for the Juliano to be fixed but hope to set sail on Sunday morning.

"Come what may, we will leave," Besancenot said.