Speaking on national television Ellen Johnson Sirleaf said some civil liberties might have to be suspended.
The Ebola outbreak has also hit Guinea, Sierra Leone and Nigeria, killing more than 930 people.
World Health Organization (WHO) experts are meeting in Geneva, Switzerland, to discuss a response to the outbreak.
The two-day meeting will decide whether to declare a global health emergency.
Ebola, a viral hemorrhagic fever, is one of the deadliest diseases known to humans, with a fatality rate of between 55% and 90%. It is spread through contact with the bodily fluids of Ebola patients showing symptoms.
A WHO statement on Wednesday said 932 patients had died of the disease in West Africa so far, with most of the latest fatalities reported in Liberia, where at least 282 have died of the virus,
Announcing a state of emergency for 90 days, President Ellen Johnson Sirleaf said in a statement that the government and people of Liberia required “extraordinary measures for the very survival of our state and for the protection of the lives of our people”.
She said that “ignorance and poverty, as well as entrenched religious and cultural practices, continue to exacerbate the spread of the disease”.
Observers say the Ebola crisis in Liberia has got worse because many people are keeping sick relatives at home instead of taking them to isolation centers.
In a surprise move, the WHO said on Wednesday it would convene a meeting of medical ethics specialists next week to decide whether to approve experimental treatment for Ebola.
Some leading infectious disease experts have been calling for experimental treatments to be offered more widely to treat the disease.
The aim of the WHO’s emergency committee meeting is to focus solely on how to respond to the Ebola outbreak.
If a public health emergency is declared, it could involve detailed plans to identify, isolate and treat cases, as well as impose travel restrictions on affected areas.
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