CONAKRY, Guinea (AP) _ President Lansana Conte told tens of thousands of cheering people Sunday that the master plotter behind last week's failed coup, Col. Diara Traore, was arrested earlier in the day ''and will be tried and executed shortly.''

Conte said he had decided that Traore, a former prime minister, and his co- conspirators were to be shot by firing squad without delay, adding:

''If anyone wants to intercede on their behalf in the name of human rights, he had better do it today, because tomorrow will be too late.''

Traore went into hiding after loyal troops crushed his attempt to seize power in the West African nation Friday, while Conte was out of the country. A national police officer told reporters Traore and 18 other leading conspirators were seized when people betrayed their hideouts during a nationwide manhunt.

Conte flew back from nearby Togo immediately after learning of the coup.

On Sunday, he addressed a mass meeting of more than 50,000 gathered in front of the People's Palace in the Guinean capital.

Conte acknowledged that his military regime took power 15 months ago with a promise not to shed any more blood following the harsh, 27-year rule of the late President Ahmed Sekou Toure.

''But it is the plotters who have shed blood, and they will be shot.'' Conte said amid renewed cheers. ''The people who died in the coup also had a right to live.''

He said 18 people were killed, including two women, and 229 were wounded in predawn shooting around Conakry radio station Friday. He said some of those killed were members of the armed forces but gave no figures. The government had claimed previously that most of the dead were violators of a curfew the rebels tried to impose after broadcasting an announcement of their coup.

Conte said he had advance information of the plot and even received a list of the top conspirators before he left Conakry last week to preside at a summit meeting of the Economic Community of West African States in Lome, capital of Togo. He implied that the government deliberately allowed the plotters to show their hand while secretly planning swift action to crush the coup when it came.

Conte said tough security measures would remain in force throughout the capital to prevent renewed looting of shops and other property belonging to Traore's majority Malinke tribe. Extensive damage was caused to Malinke-owned property, including Traore's home, immediately after the coup was foiled.

Conte is a member of the minority Soussou tribe which predominates in the Conakry area.

The capital was quiet Sunday with loyal troops and national police in position at most strategic points.

Traore, 50, spent nearly a decade in the Soviet Union under the Sekou Toure regime as a trainee officer and then Guinean military attache. He was Conte's chief companion when the two took power in a military coup in April 1984, a week after Sekou Toure died in the United States following an operation.

Conte then named Traore prime minister, but the two men split over tribal and personal rivalries eight months later and Traore was demoted to minister of education.