The gunmen, who were in a black car, fired at Sirmed's vehicle at Murree Road near Bani Gala on the outskirts of Islamabad while she was returning home from work this evening.
"They tried to target us twice and fired several shots at my car which missed. Luckily, my driver sped away and we escaped," Sirmed said.
Sirmed is an outspoken defender of democracy and human rights, especially the rights of minority communities like Hindus, Christians and Shias. She has received threats from rightwing and extremist groups several times in the past, forcing her to change her residence frequently.
Often seen wearing a sari and a bindi, Sirmed has also faced accusations of being "pro-Indian" from extremists.
However, threats have never deterred her from taking steps to protect the rights of minorities and she was recently at the forefront of a campaign to prevent the abduction and forcible conversion of Hindu girls.
Sirmed, who works as the manager of a UN project to strengthen Pakistan's democracy and parliament, also played a key role in the recent campaign to free Rimsha Masih, a Christian girl who was wrongly accused of blasphemy after a Muslim cleric planted evidence against her.
Friends who had visited Sirmed at work two days ago said they had spotted a suspicious looking car parked outside her office late at night.
Police officials said they had launched an investigation into Friday's shooting.
No group claimed responsibility for the incident.
In a message posted on Twitter, interior minister Rehman Malik said Sirmed had been provided security at her residence.
Malik said he had asked authorities to submit a report on the attack on Sirmed's car.