India claims the former naval officer was "kidnapped" from Iran and said his execution would be an act of "premeditated murder."
Kulbushan Jadhav was arrested in March last year, "for his involvement in espionage and sabotage activities against Pakistan," according to a statement released by the Pakistan armed forces Monday.
The statement said Jadhav confessed that he was tasked by India's foreign intelligence agency, the Research and Analysis Wing (RAW), to "plan, coordinate and organize espionage / sabotage activities aiming to destabilize and wage war against Pakistan."
A military court found Jadhav guilty on two counts of espionage and sentenced him to death.
India has vociferously objected to Jadhav's sentencing, saying consular officials were denied access to him during his trial, in defiance of international law.
New Delhi urgently summoned Pakistani diplomats Monday to discuss the case.
"Jadhav was kidnapped last year from Iran and his subsequent presence in Pakistan has never been explained credibly," the Indian foreign ministry said in a statement.
The foreign ministry previously claimed Jadhav, a former naval officer, was operating a business in Iran prior to his arrest in Pakistan.
Monday's statement said there was no "credible evidence" against Jadhav and described his sentence as "farcical."
If the sentence is carried out, the statement said, "the government and people of India will regard it as a case of premeditated murder."
Amnesty International said
military courts, which were used in this case, were linked to coerced confessions and unfair trials.
Pakistan executed 87 people last year, making it the world's fifth biggest executioner, according to an Amnesty report on the global death penalty this week
Jadhav was charged under the Pakistan Army Act 1952
and the Official Secrets Act 1923,
both of which provide for the death penalty.
Espionage has long been a tense subject between Pakistan and India. In 2013, Sarabjit Singh, an Indian man sentenced to death for spying, died in a Pakistan jail after being attacked by fellow inmates.
More than 40 alleged Pakistani spies have been arrested in India since 2013, according to the government.
While India does retain the death penalty, and hundreds of people were sentenced last year, only three executions have been carried out since 2007, according to Amnesty.
'Headed for crisis'
Talat Hussain, an Islamabad-based defense analyst said Jadhav's sentence could have "a very major impact" on the India-Pakistan relationship and further aggravate ties.
"I think we are heading for a major crisis," he said. "This will not bode well for both the countries and the region."
Tensions between India and Pakistan have increased in recent months over continued violence
in the disputed region of Kashmir, control of which both countries claim
In November, Pakistan evacuated thousands of people
from the parts of Kashmir administered by Islamabad, blaming "Indian shelling." That came after an attack by militants on an Indian army base left 18 soldiers dead
Last month, Indian Home Minister Rajnath Singh called for the country to completely seal its border with Pakistan
on the grounds that terrorists were using it to infiltrate the country.