An estimated 5,100 apartments in Alexandria will be renovated or demolished before 1988, and the process could force many of the city's poorest residents from their homes as early as next year, according to a report released last night.

The report to the City Council said rehabilitation is already planned or under way at 2,057 apartments.

"Housing is the most important issue," said City Council member Lionel Hope at last night's council meeting. "It's the biggest problem confronting us."

In the Arlandria section of the city, 3,000 residents will have to move next year because of developers' plans to improve the area, officials said. Because of a vacancy rate of about 1 percent, they said it was unlikely many of those residents will stay in the city.

"It has become a national problem, clearly more than a locality can deal with," said Mayor James P. Moran Jr.

He said severe federal cutbacks have made the housing problem acute and there is a five-year waiting list for Alexandria's public housing and publicly assisted apartments.

The report by the city's Office of Housing also targeted restrictions on children in rented housing.

A 1984 city survey showed that 18 percent of the 34,000 rented units did not allow children to live there. An additional 73 percent restricted the number of children.

The city's Human Rights Commission asked the council last night to consider drafting an antidiscrimination ordinance to allow more families to live in Alexandria.

If the city doesn't act soon, "families with children, particularly low- and moderate-income families, will be precluded from living in this city," said Jeanne Connell, chairwoman of the commission.

Moran told Connell that the council knew this was a big problem and said, "The only families who will be able to live here if this continues are those affluent enough to afford an expensive single-family home."

The City Council voted to schedule a public hearing Jan. 18 on restrictions barring children from apartments.