Armenia's main opposition leader has vowed to continue protesting against the result of the presidential election, which he says was rigged.
Mr Ter-Petrosian called off street protests after the clashes
Levon Ter-Petrosian made the pledge despite the deaths of eight people in clashes between his supporters and riot police on Saturday.
A state of emergency is in force in the capital Yerevan and the army has been patrolling the streets.
The election was officially won by PM Serzh Sarkisian with a big majority.
International observers judged the election to be generally democratic but noted some problems with the vote count.
Two pan-European bodies, the Council of Europe and the Organisation for Security and Cooperation in Europe (OSCE), have voiced concern about the situation in the ex-Soviet republic.
The OSCE is sending an envoy to try to arrange talks between the government and the opposition.
The New York-based Human Rights Watch (HRW) has called on Armenia to conduct a "prompt and independent investigation into the use of lethal force by security forces".
Mr Ter-Petrosian told reporters there could be years of political conflict ahead.
"Acting within the law...we will struggle... until the removal of this hateful, criminal regime... without being afraid of prison and without being afraid of their threats," he said.
Mr Ter-Petrosian expressed regret about the eight people who died on Saturday after armed police fought pitched battles with protesters who had set up barricades in the city centre, but he said the government was to blame.
The authorities said they had to restore order after 11 days of mass demonstrations which they had repeatedly warned were illegal.
Outgoing President Robert Kocharian declared a state of emergency was in force until 20 March.
Mr Ter-Petrosian was the president of Armenia in the 1990s but recently made a dramatic comeback as an outspoken opposition leader, the BBC's Matthew Collin notes.
Mr Ter-Petrosian gave his news conference on Sunday at his heavily guarded mansion on a hill-top overlooking Yerevan where he says he is being kept against his will by police.
But the authorities insist he is not under house arrest.
Official results gave Mr Sarkisian 53% of the vote in the 19 February election, with Mr Ter-Petrosian getting 21.5%.
OSCE special envoy Heikki Talvitie hopes to have talks with all the key figures concerned.
In its preliminary assessment of the election, the OSCE said last month that the vote had been "mostly in line with the country's international commitments".
However, it also noted some "problems... particularly during the counting".
HRW reported instances of intimidation at eight polling stations in or around Yerevan.
"Assailants intimidated, threatened, and even violently attacked opposition party activists, domestic observers and journalists," it said on 22 February.