BANGKOK, Thailand (AP) _ The Laotian Embassy in Bangkok today charged that ''a puppet of foreign reactionaries'' planted the bomb that exploded in Vientiane during Soviet Foreign Minister Eduard Shevardnadze's recent visit there.

Embassy press attache Ly Bon Kam phoned reporters and read a statement saying that Monday's blast outside the Soviet cultural center in the Laotian capital was designed ''to create a bad impression of Laos, to try to show that Laos was an unstable country.''

He said a Laotian who planted the bomb was killed instantly when it exploded at 5:30 a.m.

The man was a ''member of foreign reactionaries, a puppet of foreign reactionaries,'' Ly said, declining to elaborate.

The communist government in Laos has accused Thailand and other countries of supporting various guerrilla opposition groups. The groups are not considered serious security threats.

Ly said Shevardnadze had no plans to visit the center Monday. The Soviet foreign minister left Laos for Vietnam Wednesday and was scheduled to fly home today at the end of a six-nation Asian tour.

It was the communist government's first statement on the attack, although Western diplomats in Vientiane say the incident was reported in a Vientiane newspaper, without any mention of casualties.

An official of the Australian Embassy in Vientiane on Wednesday quoted Laotian police as saying the crude homemade bomb went off as the assailant was setting the timing mechanism. He said another Laotian was injured in the blast, and police were investigating if he was involved in the attack.

Ly's statement made no mention of the injured man.