Oxfordshire MEP Caroline Lucas is backing campaigners who are opposed to the detention of asylum seekers at Campsfield House.
A monthly protest outside the immigration detention centre in Kidlington was among a series of demonstrations held across Europe to oppose the locking up of refugees.
Dr Lucas described the controversial Asylum and Immigration Bill as unfair and discriminatory.
She warned that migrants would continue to be treated inhumanely.
She said: "I am supportive of those all across Europe who, on Saturday, called for the recognition of migrants rights and for the end of inhumane detention."
"The vast majority of migrants are industrious, courageous, and determined. They don't want a free ride. They want a fair opportunity.
She added: "They are not criminals or terrorists. They are law-abiding."
Members of the Campaign to Close Campsfield House have pledged to continue monthly protests until the centre, in Langford Lane, is closed.
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