President Ellen Johnson Sirleaf ordered non-essential workers not to come to work and promised that all government workers would still be paid.
Liberia's schools are already closed in the effort to keep large numbers of people from gathering and potentially spreading the disease.
The World Health Organisation says up to 20,000 people may contract the virus before it is put under control, and that it could take six months to do so.
The WHO said a student from Guinea arrived in Dakar by road on August 20 and was staying with relatives "in the outskirts of the city."
It said that on August 23, he went to a medical facility seeking treatment for fever, diarrhoea and vomiting, all symptoms of Ebola.
He was treated for malaria and continued to stay with his relatives before turning up at the Dakar hospital on August 26.
