The $50bn (Â£32bn) scheme, being built by a Chinese firm, will be longer, deeper and wider than the Panama Canal.
But the demonstrators fear it will have huge environmental costs and force thousands off their land.
Nicaragua's government says the canal will bring vital investment to one of Central America's poorest countries.
Some among the protesters, who were mostly farmers, accused President Daniel Ortega of selling Nicaragua to the Chinese.
Initial site work began last December, with completion due in five years.
Are you in Nicaragua? What is your opinion of the canal? You can share your experiences by emailing haveyoursay@bbc.co.uk.
If you are available to speak to a BBC journalist, please include a telephone number.
If you have images from the protests you can email them toyourpics@bbc.co.uk, upload them here, tweet them to@BBC_HaveYourSay or text them to +44 7624 800 100.
You can also send us your comments or pictures on WhatsApp. Our number is: +44 7525 900971.
Read our terms and conditions.
Thousands of Nicaraguans have held a protest against the planned construction of a canal linking the Atlantic and Pacific oceans.