The Central Bureau of Investigation on Sunday arrested four persons -- including three doctors -- for allegedly leaking question papers of the Combined Admission Test for the Indian Institutes of Management.

They were arrested from Shanti Palace hotel in Mahipalpur, southwest Delhi, while they were giving the papers to four candidates, the CBI said.

The agency, in a first information report, said it was a nationwide racket and the gang was involved in leaking papers of other tests.

The arrests were made when thousands of candidates were taking the exam in several centres across the country.

The CBI said raids were being conducted at five places in Patna to locate the gang's leader, identified Ranjit Singh alias Ranjit Don, who runs a pharmaceutical company in Mumbai.

The exams were cancelled following the arrests. The decision was taken after Union human resource development ministry contacted authorities at IIM, Ahmedabad, the coordinating body for the tests, which confirmed that the leaked papers matched the original, official sources said.

Fresh dates for the exams will be announced later, the sources said.

A CBI spokesperson said the gang had a "unique modus operandi".

"They would first scout for the victim and ask him to meet them at a particular place and take advance. They would charge Rs 2-4 lakh per student for helping them in the exam."

According to the spokesperson, the students were put up in a hotel a day before the exam.

"The doctors would help them in solving the question paper on an answer-sheet, which was smuggled into the exam centre. With the help of invigilators the exam sheet was replaced by the one brought by the beneficiary student," he said.

"It is going to have countrywide ramification. We are interrogating the doctors at the moment and more arrests are not ruled out," he said.

Students and their parents who appeared for the exam were angry.

Swati Sidhu, who travelled 20 km from her residence in Narela to her centre in Ashok Vihar, told rediff.com: "When I was taking my exam I came to know that the question paper had been leaked. All of us were taken aback."

Ashish Juneja said he had been preparing for the exam for almost a year.