The school says the broad campaign would help renovate undergraduate dorms, fund new research on providing low-cost energy to the developing world and expand Harvard's school of engineering, among numerous other aims.
Harvard President Drew Gilpin Faust says the campaign will help the school meet the world's increasingly complex and pressing needs.
The campaign quietly began two years before Saturday's public launch. Harvard says it has already raised $2.8 billion in gifts and pledges.
The school aims to reach its $6.5 billion goal by 2018.
If it does, the campaign would surpass a five-year, $6.2 billion campaign by Stanford University that ended last year.