Environmental leaders told the crowd the policy is likely to disrupt the marine ecosystem and put dolphins, turtles and other sea creatures at risk, the West Australian reported.
"They say sharks are dangerous, yes they are, but we choose to enter the water," said Rachel Siewert, a state senator and member of the Green Party.
Sea Shepherd, a group best known for its efforts to disrupt Japanese whale hunting off Antarctica, promised to follow Australian vessels engaged in culling sharks.
Seven people have been killed by sharks in Australia in the past three years, most of them in West Australia. Under the new government policy, large great white, tiger and bull sharks found within a kilometer (0.6 miles) of the beach would be killed and baited drum lines set out around beaches.
A similar rally was held Saturday near Melbourne in Victoria. Organizers say at least 35,000 people in Australia and elsewhere have signed petitions against efforts to cull sharks.
A spokeswoman for the West Australian government said Queensland in eastern Australia has used drum lines successfully for 50 years.
Acting West Australia, premier Kim Hames said large sharks may have become more likely to attack people.
"Humans aren't the normal prey of sharks," Hames told the West Australian. "Maybe sharks have gotten so many and so big that there will be sharks that go out and attack people when they see them."