MONTERREY, Mexico (AP) _ Floods triggered by 15 hours of heavy rain left at least seven people dead and 3,500 others homeless today in this northern Mexican city, officials said.

''The situation is very grave,'' said Red Cross spokesman Andres Castillo Perez. In some places in Nuevo Leon state, residents said floodwaters were more than four feet deep.

Castillo said three men, including a former Red Cross worker, were swept to their deaths when the normally dry Topo Chico drainage channel in the San Nicolas suburb overflowed.

Monterrey is normally so dry that in recent years poor people built tiny, concrete-block houses in the floor of the channel and along its banks.

Witnesses saw the bodies of the former Red Cross worker, identified as 26- year-old Jorge Humberto Castro Martinez, and two unidentified men floating in the channel, Castillo said.

In the Santa Catarina suburb on Monterrey's northern edge, a 19-year-old man drowned when a mound of sand he was standing on in a drainage channel gave way, police Lt. Efrain Lopez Castillo said. The man's body was recovered downstream.

Raul Garza, chief spokesman for the state government, said Thursday night area police departments reported three more deaths, but said he had no details.

A 5-year-old boy was reported missing after a nursery school was evacuated because of flooding.

Castillo said at least 3,500 residents lost their houses in the flood and were seeking shelter in Red Cross centers and local hospitals.

Officials of Nuevo Leon state said six inches of rain fell in Monterrey, about 100 miles from the Texas border, from before dawn through Thursday night. The city, the state's capital, has a population of 2.8 million.