Seychelles prefer Socialist Rene to 'Mad Mike'

International pressure seems to be a powerful reason why South Africa suddenly got tough with the mercenaries who fled here on a hijacked airliner after their failed attempt to overthrow the government of the Seychelles six weeks ago.

For weeks it looked as if most of the men would get away without even a reprimand. Only the five alleged ringleaders faced relatively minor charges for ''kidnapping.''

But last weekend arrest warrants for violating the country's stringent Civil Aviation Offenses Act were issued to all 45 mercenaries who landed on the hijacked jet at the airport in the east coast city of Durban Nov. 26. Three of the four charges each man faces carry maximum jail sentences of 30 years.

Natal Province Attorney General Cecil Rees went out of his way Jan. 5 to emphasize the gravity of the charges and the fact that they incorporate sanctions against hijacking outlined in conventions signed at The Hague, Montreal, and Tokyo.

Specifically, the men are charged under laws which make it an offense to ''seize an aircraft by force,'' perform an act that could ''jeopardize an aircraft or airport,'' or possess any ''harmful article on an aircraft or at an airport.''

The case is expected to be heard in full in the Supreme Court in Natal, starting perhaps as soon as the end of this month. It may climax one of the most astonishing affairs in South Africa's recent history.

The Seychelles caper started dramatically when the big Air India Boeing jet landed inDurban at dawn with mercenaries on board. One of them was believed to be a wounded American. There was also the body of another mercenary who had been killed in the shootout that preceded the mercenaries' departure from the Seychelles.

To the 65 paying passengers, who had been on their way from Zimbabwe to Bombay when the plane was obliged to turn back to Africa after refueling, and the 14 crew members, it seemed to be a clear case of hijacking - though many remarked that the hijackers had been ''polite.''

The mercenaries were hustled to Pretoria, the country's capital, for interrogation. Authorities were silent about the identity of those under arrest but it soon leaked out that one of them was the legendary Col. ''Mad Mike'' Hoare, an Irishman by birth but a longtime resident of South Africa, who led a band of mercenaries in the Congo during its civil war.

Then 39 of the men were released and only the five, including Hoare, were charged. An outcry in South Africa and abroad at this apparent complete contradiction of South Africa's hardline attitude toward hijacking and terror attacks appears to have provoked some governmental rethinking.