In 2015, Armenians voted in a referendum to shift the country from a presidential to a parliamentary system, stripping powers from the president and giving them to the prime minister.
Thousands of opposition supporters had spent the day rallying outside the parliament building to support Pashinyan.
56 deputies from the RPA faction voted against, two did not take part in the voting.
While addressing fellow lawmakers earlier in the day, Pashinian warned that Armenia would be struck by a "political tsunami" if he were not appointed as prime minister.
"It is very important to enhance the effectiveness of the mechanism of property declaration by officials, since now this mechanism gives them room for hiding information about property", Pashinyan said.
Pashinyan vowed after the election to continue the protests, calling on supporters to block roads on Wednesday.
Immediately after the vote, Pashinyan rushed to join the thousands now gathered in Republic Square who were anxiously awaiting the election results.
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In the vote taken today at the National Assembly's special session, 45 votes were cast in favor of Pashinyan, while 55 votes were cast against.
But the ruling Republican Party, allied to Sarksyan, has a majority in the legislature and after hours of acrimonious debate it withheld its support for Pashinyan's candidacy, leaving him short of the support he needed.
During the parliamentary debate, Republican Party lawmakers accused Mr Pashinyan or being an irresponsible rabble-rouser, they alleged he recruited children to join his protest movement, and said he lacked the qualities to command the Armenian armed forces.
"Given the meeting we had yesterday, I am sure Mr. Pashinian can not be the prime minister", Sharmazanov told reporters on May 1. The Tsarukyan alliance has 31 seats, and the Yelk faction has 9 seats. The ARF faction - (Armenian Revolutionary Federation aka Dashnaktsutyun), has 7 seats.
For the post of Prime Minister nominated only one candidate Nikol Pashinian.
"We will stay here and wait for the instructions of Nikol Pashinyan", TASS cited him as saying.
But Mr Pashinyan has indicated the party may seek to halt his bid.
When it became clear they would not support him, Mr Pashinyan told parliament the Republicans had "declared war on the Armenian people". Mechanisms of the early elections should be devised as soon as possible. "It would be the final stage of the people's victory", he said in parliament.