It offers Europeans in the UK fewer rights than Britons in the EU, they say in a joint letter to newspapers.
Mrs May has said about three million EU citizens would be allowed to stay.
EU migrants who had lived in the UK for five years would be granted access to health, education and other benefits.
But the prime minister's proposal's would be dependent on EU states guaranteeing Britons the same rights.
The leaders of the four political groups who have signed the joint letter account for two-thirds of the votes in the European Parliament.
Their letter points out that that they have the power to reject any Brexit deal before it can go ahead because the parliament must approve the withdrawal agreement.
The leaders said they would not endorse anything that removed rights already acquired by citizens.
They said the UK proposal "falls short" because it would take away rights citizens currently have, and create new red tape and uncertainty for millions of people.
The letter said this contradicted promises made by the Leave campaign that EU citizens would be treated no less favourably after Brexit.
By contrast, the letter said the EU's offer - already on the table - was simple, clear and fair because it promised that all citizens, including UK nationals living in Europe, would be treated equally and lose no current rights.
'Rights and values'
The letter stated: "The European Parliament will reserve its right to reject any agreement that treats EU citizens, regardless of their nationality, less favourably than they are at present.
"This is a question of the basic fundamental rights and values that are at the heart of the European project."
It added: "In early 2019, MEPs will have a final say on the Brexit deal.
"We will work closely with the EU negotiator and the 27 member states to help steer negotiations."
BBC Europe correspondent Damian Grammaticas says it is clear there is deep scepticism in the European Parliament about Mrs May's offer on citizens' rights.
'Unhelpful distortion'
A spokesperson for the UK government said: "The position presented to European Parliament is an unhelpful distortion of an offer that several member states have already said is the basis of a good deal.
"Regrettably, the article includes a number of inaccuracies that are particularly unhelpful and likely to cause unnecessary and needless concern to UK and EU citizens.
"We have always said we want a reciprocal arrangement that allows EU citizens to continue to live their lives in the UK broadly as they do now, which is why we set out our proposal to guarantee their rights with a new 'settled status'.
"That means EU citizens with settled status will have access to education, healthcare, benefits, pensions and social housing on the same basis as British citizens under proposals enshrined in UK law."
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