One in three is paid less than €600 per month, according to the survey.
A further 10 per cent rated the level of tutoring from the MEP as "poor" or "very poor."
The vast majority were working between 36 and 40 hours.
The survey was conducted by Parliament on the issue of unpaid interns working in the EU. Young people working for MEPs and political groups were asked about their pay and conditions.
Nearly 250 interns took part in the survey, 54 per cent of them female.
Details were released at a special event on the issue in Parliament earlier this week.
Among those taking part were German Greens MEP Terry Reintke, who has been one of the main deputies raising concerns about the issue, and Bryn Watkins, co-founder of the Brussels Intern NGO B!NGO, who are fighting for paid internships.
The campaign is also backed by the European Parliament Stagieres Association and Parliament's youth intergroup.
Reintke said that the youth intergroup had already started a Fair Internships campaign and it was not only about unpaid interns in the Parliament but in the other EU institutions as well.
The event was used to premiere a new documentary, 'Colours of unpaid youth' about unpaid internships inside the EU institutions and other international organisations.
Reintke said, "It is important to us to show this movie and to show the experiences of the young people. This is a problem we need to pay attention to."
She said that 41 MEPs had signed a manifesto on quality internships.
"We want to keep the pressure in order to make a change."
Earlier this year, some interns staged a protest action outside the Commission in Brussels to highlight their plight.
The EU ombudsman also recently criticised the EEAS over its policy on interns.
Bryn Watkins, co-founder of the Brussels Intern NGO B!NGO, said, "We have been really heartened in recent weeks by the excellent turnout for the interns strike, the ombudsman ruling against unpaid internships in the EEAS, and the launch of the manifesto for paid internships from the European Parliament youth intergroup.
"The moral case against unpaid internships has been won, and it is great to see that employers are beginning to wake up to that. However, there is still far too much hypocrisy in the EU institutions, which claim to fight for young people while also taking hundreds of unpaid interns every year.
"We will be working with our allied to build on the ombudsman ruling and call for an end to unpaid internships in all EU institutions."
A B!NGO source said, "It is estimated that in Europe alone there are over three million young people working as unpaid interns per year, equivalent to the entire workforce of Denmark. Europe's young people have carried the heaviest burden of an economic crisis they did not cause. Recovery is slow, and unpaid internships are not part of the solution.
"Unpaid internships discriminate against people from poorer backgrounds and widen social inequalities. They undermine the tax system and leave young workers without social security and protection at work. They create a culture where employers do not train their staff, seeing no reason to invest in them."
He added, "This is bad for the whole economy, as skills are wasted and real jobs are replaced by precarious poor-quality internships."
Allan Pall, Secretary General of the Youth Forum, said, "Unpaid internships, zero hour contracts and other forms of precarious work are trapping young people in poor quality employment and wasting their potential. We stand alongside these young people speaking up for their most basic workers' rights."