Williams said the unit housed 29 patients who were receiving preliminary treatment before being taken to hospital. It was unclear how many are now at large.
"They had all tested positive for Ebola," he said, adding that nine had died, without elaborating.
Wesseh said she heard the assailants shouting that President Ellen Johnson Sirleaf "is broke", adding: "She wants money. There's no Ebola" in Liberia.
Wesseh said the mostly young men armed with clubs broke into the isolation unit set up in a high school in a Monrovia suburb.
Nurses also fled the attack, Wesseh said.
The Ebola outbreak, the worst since the virus first appeared in 1976, has claimed 1,145 lives in five months, according to the UN World Health Organization's latest figures as of August 13: 413 in Liberia, 380 in Guinea, 348 in Sierra Leone and four in Nigeria.