MANAGUA, NICARAGUA, FEB. 9 -- A hand grenade apparently hurled by a rebel fighter at a Sandinista street march killed nine Nicaraguans and injured 32 in the northern mountain town of Wiwili Saturday night, American witnesses and victims' relatives reported today.

The attack brought to 28 the number of people said killed in the past week by the rebels in the northern province of Nueva Segovia. Almost all were civilians.

The other 19, including five children, were killed near Wiwili's neighboring town of Quilali on Thursday when uniformed men believed to be contras -- as the rebels are known -- blasted a brightly painted civilian passenger truck with rifle fire and a powerful shrapnel mine.

Townspeople, in interviews over the weekend, said those deaths made them recoil from the rebel cause and look favorably on the Feb. 3 congressional vote in Washington against military aid for the contras.

Quilali has been sharply divided between those loyal to the government in Managua and those favoring the contras. Dozens of its youths are in the contras' ranks. Both sides followed the extensive radio coverage from Managua of the vote in Washington.

"It brought us a little bit of relief to know that the U.S. Congress didn't approve more funds for death," said Baptist Pastor Camilo Sevilla in Quilali, a town of about 4,500.

The attacks also raised popular fears in the northern mountain region that disappointed contra fighters may become less disciplined in their operations and more violent toward civilians following the cutoff of military aid.

The Wiwili attack killed five children under 16 years old, and wounded 21 children, according to the American religious human rights group Witness for Peace, which monitors civilian casualties in contra attacks and openly opposes the contras. Two volunteers in Wiwili, about 180 miles north of the capital, radioed the organization's Managua headquarters in the past two days with the information.

Sandinista authorities in Wiwili organized a march of about 50 residents late Saturday to protest the mine attack near Quilali two days earlier. The marchers, shouting anticontra slogans, wound down a path along the edge of town, with many children tagging along. Four demonstrators were wearing olive-green Sandinista uniforms, Witness for Peace reported.

"Suddenly I heard a huge explosion that threw all of us to the ground, some bleeding, some dead. That's the last thing I remember," said a 16-year-old survivor, Juan Carlos Moreno, interviewed in a military hospital in the northern city of Jinotega by a reporter from Barricada, the Sandinista daily.

A grenade was tossed out of the darkness from tall weeds on the edge of town, said Alba Lidia Diaz, wife of Sandinista Youth leader Jose Garcia. He was hospitalized in Jinotega with severe shrapnel wounds. In Wiwili, police said they found a grenade pin near the site. The Interior Ministry said two unidentified Nicaraguan suspects are being held.

"There's not much doubt the contras did it, since all the organizations in the march were Sandinista," Diaz said in an interview as she waited for news of her husband at the Jinotega hospital.

Bosco Matamoros, a spokesman for the Nicaraguan Resistance, the main contra alliance, denied that the contras had any role in either of the two recent attacks.

"We've consulted with our commanders and none of them has carried out any action of this kind. We consider these terrorist acts," Matamoros said in a telephone interview from another Central American country. Speaking on the condition that his location not be published, he claimed the Sandinista Interior Ministry committed the attacks in a "macabre logic of terror" to blame them on the contras.

Since late December, contra forces have laid many ambushes for military vehicles and state-owned tractors near Quilali, in operations seemingly intended to slow or halt traffic on the main highway through Nueva Segovia province.

On Jan. 30, a contra field commander who said his name was "Fiera" stopped and searched a truck loaded with civilian passengers on the Quilali road. One of the passengers, Witness for Peace volunteer Erik Nicholson, said he saw that contra guerrillas had set up an ambush on either side of the road. On a bank to one side, Nicholson said he spotted an olive-green box with two wires attached that appeared to be a remote-controlled mine.

Since a series of mine attacks by contras against civilian vehicles in mid-1986, contra fighters no longer carry mines triggered by vehicles' weight, Matamoros said. But they continue to be armed with claymore mines, activated by a rebel near the site of an ambush.

Several survivors of the truck stricken near Quilali said many of its passengers had relatives fighting for the contras.

One victim, Jose Esteban Talavera, was identified by Matamoros as the father of a veteran contra field commander code-named Chacal -- who earlier lost a leg to a Sandinista mine. Matamoros said a second Talavera son is a Redeye missileman for a contra regional command that carries the name of the town of Quilali.

"Quilali is one of the towns that have given us the most support. We have a deep popular base there," Matamoros said.

A survivor from the mined truck, 20-year-old Maria Ofelia Peralta, said her 3-month-old son Juan Carlos was killed in her arms by shrapnel pellets. Peralta was one of several mothers who were bringing small children back to Quilali from a nearby hospital, where they had been treated for illness and released the morning of the mining attack.

Housewife Norma Martinez, an evangelical Christian, said her husband Felix was killed, leaving her with five children to raise. "God grant us that the United States will stop giving aid to the people who did this," she said.