The demonstration on Thursday came three days after the reform package was adopted alongside the 2015 budgets of the National Health Insurance Fund and the National Social Security Institute during a cabinet meeting chaired by Borisov.
The Confederation of Independent Trade Unions, one of the two largest unions in the country which organized the demonstrations, said the government was "raising the retirement age and freezing salaries".
Thousands of protesters from all parts of the country marched along the main streets of the capital shouting; "we are strong" and "we do not want to work until the end of our days".
Under current legislation in Bulgaria, the retirement age for manual workers is 63 years and eight months for men, and 60 years and eight months for women.
Nearly a third of Bulgaria's population of about 7.3 million people are pensioners.
The government says the pension reform will make Bulgaria more "competitive" and its pension system "sustainable".
More than 15,000 workers who were due to retire next year will be directly affected by the pension changes