At least 30 arsons have been reported since the beginning of 2008, about half of them in the last three weeks. Police said the blazes may be part of a gang initiation, but there was no clear information who was committing the crimes or why.
The latest fire was reported late Saturday at the rear of one house and quickly spread to adjacent homes in the Chester County community.
"This is an arson, no question about it," City Manager Harry Walker said Sunday.
The emergency declaration gives the city powers to deal with the situation without worrying about the budget, such as boarding up the buildings, assigning police to protect them and helping the families involved, Walker said.
The fire came despite stepped-up police patrols and investigative help from county, state and federal agencies. Three people were arrested in December.
"A lot of people are scared," resident Marissa Martinez said as she watched smoke rise from the rubble. "I never thought things could come to this point."
Fifteen homes were damaged and some may have to be demolished, Walker said. Damage was estimated at $1.2 million, bringing the total fire damage since last summer to $3 million.
The southeastern Pennsylvania chapter of the American Red Cross was helping 14 families, a total of 50 people - 32 adults and 18 children, spokeswoman Denise Venuti Free said in a statement Sunday.
One of the homes destroyed belonged to City Councilwoman Robin Scott, who said she and her family got out safely after police officers knocked on doors to alert residents.
"To see it all happen the way that it did was devastating and I wouldn't wish that on anyone," Scott said through tears to a gathering of citizens and city officials Sunday night. She urged residents to be vigilant in their neighborhoods.
Police Chief William Matthews said more than one person appears to be involved because of the number of fires and the fact that many have occurred within minutes of each other.
Walker said authorities fear that the latest blazes were copycats, since they had already arrested three people in December believed to have been responsible for 15 fires.
"The more we caught them, the more fires were set," he said.
Authorities urged residents to remove flammable materials from porches and to keep porch lights on at night or install lights that are turned on by motion detectors.
"It costs 76 cents a week to keep your porch light on," the police chief said Sunday. "That 76 cents should be considered a down payment on the safety of your family and friends in the neighborhood."
One firefighter sustained an ankle injury, but no other injuries were reported.
Coatesville is about 45 miles west of Philadelphia.