Riot police poured into the centre of the city on Saturday
Thousands of supporters of defeated presidential election candidate Levon Ter-Petrosian were involved in clashes with police and troops on Saturday.
They finally dispersed after a message from Mr Ter-Petrosian was read out.
Protests had continued for 11 days over alleged rigging of the vote which gave a convincing win to PM Serzh Sarkisian.
International observers judged the 19 February poll in the ex-Soviet Caucasus republic to be generally democratic.
Correspondents say the unrest has risked destabilising a key Russian ally which lies between the energy-rich Caspian Sea and the markets of Europe.
'Over the edge'
The message from Mr Ter-Petrosian, who says he is under house arrest, read: "I do not want any victims and clashes between police and innocent people."
Reports say vehicles were burnt and some shops looted
The state of emergency declared by outgoing President Robert Kocharian bans public gatherings and imposes restrictions on media reports.
It is to remain in force until 20 March.
President Kocharian told reporters that events had gone "over the edge".
"I appeal to the people of Armenia to show restraint and understanding," he said.
Official results gave Mr Sarkisian 53% of the vote, with Mr Ter-Petrosian, a former president, getting 21.5%.
Shots fired
The centre of the capital saw pitched battles between security forces and protesters.
We could see red tracer bullet fire being shot in two directions
Alan, witness
Eyewitness: Yerevan violence In pictures: Yerevan protests
Demonstrators hurled stones and petrol bombs, setting vehicles on fire, and police and troops responded with truncheons.
Reports say tracer bullets were also fired over the heads of demonstrators.
No official data on injuries and arrests among the protesters were given immediately but police say eight of their officers were injured, some seriously.
Eyewitnesses say one person was killed, possibly by a ricocheting bullet.
It "must have ricocheted as police were shooting in the air to disperse an opposition rally being held nearby", one demonstrator was quoted as saying by Reuters news agency.
A young BBC news website reader who attended the protests says she saw the body of a dead boy being taken away by ambulance.
"We saw a 12-year-old boy killed," she said.
"He was lying there, not moving. I don't know if he had been shot. That was about eleven o'clock [1900 GMT] this evening."
The same reader said she and others were chased by troops.
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Archivo:Robert Kocharyan.jpg|thumb|left|200px|Robert Kocharián en 2001
El presidente armenio Robert Kocharián declaró un estado de emergencia en Armenia el sábado, después de que uniformados de la policía usaran gases lacrimógenos y armas de fuego para dispersar las protestas contra fraude electoral en las elecciones presidenciales que tuvieron lugar el pasado mes de febrero, dejando como saldo 8 personas muertas en los disturbios entre la policía y la oposición.

El anuncio vino desde la oficina del presidente Kocharián poco después de que la policía suprimió la marcha de más de 15,000 manifestantes. Más temprano, las manifestaciones en tiendas cercanas a la alcaldía de la ciudad de Ereván fueron evacuadas en forma violenta usando bastones de mando.

"Ellos atacarón un demostración pacifica en Ereván," dijo el representante de la oposión :Alexander Arzumanian. "Pero la gente esta muy decidida y lucharan por sus derechos ... Hay mucha gente herida".

Los testigos describieron a Associated Press como la gente era atacada en la acción policial durante la noche, aunque no hay informes oficiales sobre muertes ocurridas en este hecho. El ministerio de salud dijo que por lo menos 30 personas, incluyendo 6 oficiales de policía, han entrado en tratamiento.

La oficina de Kocharián dice que el estado de emergencia permanecerá durante 20 días. La medida impone una censura a todas las manifestaciones y ordena que los informes de noticias de Armenia emitan solo información oficial.

Armenia, oficialmente Republica de Armenia, es un país montañoso, multi-partidista, y democrático. Permanece técnicamente en una guerra con su país vecino Azerbaiyán sobre la independencia ''de facto'' de la república de Nagorno-Karabakh.