Authorities said they would allow Greenpeace activists to protest but stepped in when the group went back on an agreement not to interfere with the tanker, RT.com reported.
Willcox and the other detainees were released by Dutch police, RT.com said.
Willcox and his crew on his previous ship, the Arctic Sunrise, were detained for 100 days by Russian authorities after staging a protest near an oil rig in the Arctic Ocean last year.
Click here to read the full story of the protest in Rotterdam.
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