French and British leaders cautioned that Turkey faced at least 10 years of changes if it was to meet EU criteria in 35 policy areas that included minority rights and legislation.
In Paris, French President Jacques Chirac said the Muslim-majority country must undergo a "major cultural revolution" to fulfil EU membership conditions. "Will it succeed? I cannot say," he said.
And UK Prime Minister Tony Blair, who holds the EU presidency, said on Wednesday that negotiations would take a long time and would mean a "very big change" for Europe and Turkey.
"It will be an issue of controversy for years to come," he told reporters in London.
Entry talks could force Turkey next year to open its ports and airports to Greek Cypriot ships and passenger jets.
This may be a first step towards recognising the EU member that Turkey invaded and occupied in 1974 after a short-lived coup by supporters of union with Greece.
Turkey has refused to recognise Cyprus, which has been separated into a Greek Cypriot south and Turkish Cypriot north, where a breakaway state is only recognised by Ankara.
Turkey formally began membership negotiations with the EU on Tuesday, after the 25-nation bloc's members held two days of arduous talks to overcome objections by Austria.
Prime Minister Recep Tayyip Erdogan warned on Tuesday that reforms "in particular will try us and there will be a great struggle to fully implement the harmonisation laws".
Turkey's prime minister talked of
a struggle to implement reforms
Nevertheless, the daily newspaper Milliyet, one of the country's largest, headlined that Turkey now had a "new life with the EU".
"Nothing will be the same from now on. We will experience changes in our lives," it said in a front page commentary.
The daily Cumhuriyet splashed "era of conditions begin" across its front page, and wrote that "with the decision to start negotiations, Turkey is entering a difficult process. During the negotiations, foreign policy, the environment and agriculture sector issues will be especially arduous".
But former ambassador and foreign policy analyst Gunduz Aktan questioned if the beginning of talks was "salvation or inquisition".
"Accession talks constitute the most effective process the EU uses to effect change in the candidate country," he wrote in the Turkish Daily News.
"In that case the demands made and the pressure exerted on such a country to change itself could go to extremes, acquiring an irrational, tortuous nature.
"The 3 October experience has shown that Turkey cannot tolerate such a thing."