GENEVA (AP) _ Swiss police said Wednesday they arrested a suspect in the killing of former Iranian Premier Shapour Bakhtiar and stepped up the hunt for an alleged accomplice in the Aug. 6 assassination.

Geneva Police Chief Laurent Walpen told reporters a routine patrol spotted the suspect, Ali Rad Vakili, wandering on a Lake Geneva beach shortly before dawn.

Walpen said he was one of the three Iranians sought by French police after Bakhtiar was killed at his home outside Paris.

He said investigators determined that a second suspect, Mohammed Aazadi, parted company with Vakili before they both crossed the border illegally into Switzerland. Both surfaced separately in Geneva where they spent two nights at different hotels before vanishing again Aug. 15, according to Walpen.

Vakili had since been wandering from place to place, sleeping in boats anchored at the marinas around Geneva. But Walpen said he had ''more money on him than a vagrant would have.''

Vakili, who apparently speaks only Farsi, is being held only on a charge of illegal entry pending the arrival of French investigators, police said. Walpen said Vakili has not been questioned about the assassination.

Swiss authorities said they expect a French extradition request to be filed shortly. Walpen said a nationwide manhunt continued in Switzerland, including the search of an Iranair jetliner before it took off from Geneva for Tehran late Tuesday.

The Iraq-based Iranian Mujahedeen opposition group has claimed that Bakhtiar, an outspoken critic of the Shah named premier only days before the overthrow of the ruler, was killed by agents of the Tehran government. Iran has denied any involvement.

Walpen said police were also looking for the man who booked the Geneva hotel room for Aazadi. But he stressed that this man may have nothing to do with the assassination.

''The arrest proves that Geneva is not a turnstile for terrorists,'' Bernard Ziegler, head of the cantonal (state) justice department, said at the news conference.

Swiss and French police have been stung by criticism that they bungled the hunt for the suspected killers of Bakhtiar, whose body was found only on Aug.8.

One day earlier, Vakili and Azadi had been briefly detained by Swiss police when they tried to enter Switzerland with forged visas in their Turkish passports.

They were fined $133 and returned to French authorities, who then released them because their French visas were valid.

After Vakili finally managed to cross into Switzerland, he registered for two nights at a Geneva hotel with his Turkish passport as Musa Kocer. A computer failure prevented police from knowing he was the same man who had been detained at the border.

There has been no trace reported yet of another suspect, identified as Farydoun Boyer Ahmadi, whose car was later found in Paris with traces of blood in it.