A plane carrying $6 million in ranson money and six prisoners left Tokyo this morning on a Japanese government mission to rescue the hostages being held by armed political radicals in an airline in Dacca, Bangladesh.

The shackled prisoners were marched into the tail entrance of a Dc-8 and a large delegation of Japanese officials entered the front for the eight-hour trip to Dacca.

The money and the prisoners were demanded by five japanese Red Army hijackers who seized the plane Wednesday over Bombay and have kept more than 140 hostages under guard at the Dacca airport. The hijackers reportedly have wired explosives to the doors of the parked airliner.

The hijackers had threatened to begin killing their hostages one by one at Bangladesh time (6 p.m. EDT if their demands were not met by 4 Friday). The ransom plane was not expected to land at Dacca until more than seven hours after the deadline, but the hijackers agreed to extend the time limit after Bangladesh authorities informed them by radio that the ransom plane was on the way.

Shortly after the rescue mission took off at 6:05 a.m. (5:05 p.m. EDT Friday), the hijackers announced the next stage of their plans in a statement received in Beirut, Lebanon.

It said that when the ransom money and freed prisoners are delivered in Dacca, all hostages except Japanese males will be released. Then the hijacked plane will be flown to an undisclosed country with the remaining hostages aboard.

All foreigners and all women and children will be released in Dacca, the Japanese Red Army statement said, according to the Kyodo News Service.

Where the hijacked plane will land once it leaves Dacca is a mystery. Japanese officials said they have contacted 10 nations for landing permission and all have refused.

At least 10 captives have been released, most because of illness, leaving 140 passengers and a crew of six on the Japan Air Lines plane in Dacca.

The six prisoners were brought from various jails in Japan and marched one-by-one aboard the plane under heavy guard in the pre-dawn darkness. The hijackers initially had demanded the release of nine prisoners, but three refused the offers of freedom.

Four of the six on the plane are political activists jailed oncharges of performing terrorist acts, such as bombings and attacks on embassies in 1974 and 1975.

The other two are described as common criminals, convicted of murder and robbery-murder. Their release was sought, it is believed, because they have been cellmates of other Japanese radicals now at large.

The Japanese government has come under considerable criticism in the past two days for its prompt acceptance of the hijackers' demands.

Groups of right-wing demonstrators protested yesterday what they described as the government's "weakness."

Police and prosecutors also have objected to the release, declaring that it will foster disrespect for the law. Ironically, the prisoners left their jails on the Japanese "Law Day," designated to encourage respect of the national system of justice.

Justice Minister Hajime Fukuda said he accepts ful responsibility for their release as necessary to save human lives and Prime Minister Takeo Fukuda had declared that "human life is more precious than the earth." The justice minister had indicated he will resign when the affair is concluded.

Leading the 45-member government delegation on the rescue plane was Vice Minister of Transportation Hajime Ishii. He said the delegation's goal "is to protect the lives and security of the people" held in Dacca. He said he is willing to become a hostage in order to free the passengers and crew members.

The plane left Tokyo's Haneda Airport carrying six tons of luggage, food, medicine and oxygen supplies. Several officials of Japan Air Lines also were aboard.

The man, identified as Jacques Robert, shot a stewardess in the arm and demanded radio air time to broadcast a tape-recorded statement in return for the release of the passengers and crew members. He also wanted the plane refueled to fly him to an undisclosed destination.

Police sources said Robert, 43, was a mentally disturbed individual who had held up Paris radio station at gunpoint in 1974.