Yes, you will have to write the JAR ATP exams. The thing in europe is that you will also have to attend a ground school. The on-site ground schools usually take 7 months, 5 days a week, 8hrs a day! U can also attend a distance-learning grounds school where you study at home, but they take 9-12 months or so.
After that you will have to do a flight test.
If you got a lot of hours might be able to skip the groundschool though:
Exemption G1.5 Note 1:
If applicant has a minimum of 1500 hours' flying experience as PIC or Co-Pilot on Multi Pilot aeroplanes, holds a valid multi pilot type rating for the aircraft to be used in the skills test and has a minimum 500 hours on this type of aircraft, he/she will be exempt from the study of the ATPL exams prior to taking the exams and passing them. In this case the exams can be taken and passed at the applicant's discretion. The skills test in this case is taken on the aircraft (or simulator) on which the applicant is type rated.
Exemption G1.5 Note 2:
If an applicant has a minimum of 3,000 hours on public transport aircraft over 3,000 kg on scheduled international routes including 1,500 as captain, he/she may apply to the authority for an assessment. Additional credits may be given by the issuing authority in this case for the Theoretical Knowledge except Air Law and Human Performance.
(Note: I've found those to excemptions on a flight schools webpage, not in the law, but I guess that's where they're taken from)
/S