Armenian investigators said Wednesday that opposition leader Vazgen Manukyan had been charged with making calls to overthrow the constitutional order after he urged the army to rebel against the current leader.

The small South Caucasus nation has been in the grip of a political crisis, with the opposition calling on Prime Minister Nikol Pashinyan to resign over his handling of last year's war with Azerbaijan.

Manukyan, a 75-year-old veteran politician who has been put forward by the opposition to replace Pashinyan, has been ordered to report to investigators on Thursday morning.

"Charges have been filed against Vazgen Manukyan," Armenia's Investigative Committee said in a statement released to AFP, adding he was accused of inciting violence and making calls overthrow the constitutional order.

Investigators said a criminal case had been opened after Manukyan had last month urged supporters to join protests and called on the army to rebel.

Last week Pashinyan defied calls to resign and accused the military of an attempted coup.

A coalition of opposition forces decried Manukyan's "political persecution" and said it would not influence "his principles, political struggle and resolve to prevent a catastrophe threatening our country."

In 1990-1991, Manukyan was post-Soviet Armenia's first prime minister and also served as defence minister in 1992-1993.

Pashinyan has faced fierce criticism since he signed a peace deal brokered by Russia that ended the conflict over Nagorno-Karabakh, an ethnic Armenian region that broke from Azerbaijan's control during a war in the early 1990s.

Fresh fighting erupted over the region in late September with Azerbaijani forces backed by ally Turkey making steady gains.

After six weeks of clashes and bombardments that claimed some 6,000 lives, a ceasefire agreement was signed that handed over significant territory to Azerbaijan and allowed for the deployment of Russian peacekeepers.

The agreement was seen as a national humiliation for many in Armenia.