More than 5,000 people including civilians were killed in Armenia and Azerbaijan when clashes erupted between the ex-Soviet enemies in late September over the breakaway region of Nagorno-Karabakh.
The war ended in November with a Moscow-brokered peace agreement that saw the Armenians cede swathes of territory to Azerbaijan, which has been backed by close ally Turkey.
The deal sparked fury in Armenia, with the opposition urging Prime Minister Nikol Pashinyan to resign.
On the first day of the national mourning on Saturday, Pashinyan was expected to lead a procession to a memorial complex in the capital Yerevan where victims of the conflict are buried.
The opposition planned to hold a separate march later in the day. Pashinyan's critics have called on supporters to stage a national strike, starting Tuesday.
"The entire nation has been through and is going through a nightmare," Pashinyan said in a video address ahead of the memorial march.
"Sometimes it seems that all of our dreams have been dashed and our optimism destroyed," he added.
The 45-year-old former newspaper editor was propelled to power in 2018 after he channeled widespread desire for change into a broad protest movement against corrupt post-Soviet elites.
But after six weeks of clashes with Azerbaijan, many have called Pashinyan a "traitor" for agreeing to what they say is a humiliating deal with Azerbaijan. He has so far refused to step down.
As part of the peace deal Russia deployed nearly 2,000 peacekeeping troops to Karabakh.
Moscow said on Friday that a Russian mine clearer was killed by a blast in Karabakh when an explosive went off earlier this week.