The European Commission - focus of much of Baroness Thatcher's political venom in the 1980s - described her as a "circumspect yet engaged player" in the EU, after the former British leader died earlier on Monday.
Commission President Jose Manuel Barroso hailed her as a great stateswoman and highlighted her role in bringing central and eastern Europe into the EU "family" after the collapse of communism and the fall of the Berlin wall.
Mr Barroso, speaking at Commission headquarters in Brussels, said: "Let me, on behalf of myself and the European Commission, say that I was deeply saddened to hear the news of the death of Baroness Thatcher.
"She was without doubt a great stateswoman, the first female Prime Minister of her country, and a circumspect yet engaged player in the European Union. She will be remembered for both her contributions to and her reserves about our common project.
"She signed the Single European Act and helped bring about the Single Market. She was a leading player in bringing into the European family the Central and Eastern European countries which were formerly behind the Iron Curtain. As you remember, Britain under Mrs Thatcher's leadership was very supportive of the enlargement of the European Union."
Mr Barroso added: "Her legacy has done much to shape the United Kingdom as we know it today, including the special role of the United Kingdom in the European Union that endures to this day. I would like to convey my deepest regrets to the Government and people of the United Kingdom."
The President of the European Parliament, Martin Schulz, called her a "committed European" - at least at the beginning of her premiership.
"Margaret Thatcher marked British and European political life. Despite our clear political differences, Margaret Thatcher is a figure of historic significance.
"Margaret Thatcher at the beginning of her tenure was a committed European, signing and pushing for the single European act which transformed the EU single market.
"No matter whether one agrees with her policies or not, Margaret Thatcher showed that politics still has the capacity to be a force for change."