Organisers said they wanted to "show support" for the campaign against drilling for shale gas in Lancashire.
About 35 people broke through fencing into the site near Preston New Road, before they "left of their own accord", BBC Radio Lancashire reported.
Francis Egan, chief executive of the fracking company Cuadrilla, said it was "unacceptable and irresponsible".
Lancashire Police said no arrests were made but the demonstrators who broke in were "thought to be from outside the area and not from the local protest groups".
The government's approval for fracking at the Little Plumpton site is set to face a judicial review next month.
Cuadrilla has previously said drilling would start in the spring.
It would mean that, for the first time, UK shale rock will be drilled horizontally, which is expected to yield more gas. However, the process has prompted environmental concerns.
"It's more important now than ever that we send a strong message... that there is no social licence for fracking in Lancashire or anywhere," organisers from Frack Free Lancashire said.
Protests have been held daily, they said, since work to prepare the site for shale gas extraction started on 5 January.
Earlier in February, concrete firm Moore Readymix terminated a contract with Cuadrilla following protests at its depot.
A spokeswoman for Cuadrilla said: "Work at our site is progressing well and we thank all those who continue to support our operations, which is a great opportunity to create jobs, fuel businesses, heat UK homes and stimulate economic growth in the region.
"For those who remain unconvinced about shale gas's role in securing our nation's energy future, we of course respect the right they have to protest peacefully."
The company has not yet been given permission for work at a second Lancashire site - Roseacre Wood - amid concerns over the impact on the area.
Lancashire County Council had initially refused permission to extract shale gas at both sites on grounds of noise and traffic impact but the government overruled the decision for the Preston New Road site.
About 250 people have attended a rally near the UK's first horizontal fracking site, police said.