An estimated 3,000 people packed the Iolani Palace grounds Sunday afternoon for a rally that marked the largest gathering on the issue on Oahu to date as opposition continues to intensify against construction of a massive telescope atop Mauna Kea on Hawaii island.

Organizers said the event allowed Oahu residents to show solidarity for the so-called protectors camping out on the mountain to halt construction of the Thirty Meter Telescope, a $1.4 billion project to erect one of the world’s largest and most powerful optical telescopes.

Many Native Hawaiians view the volcano as a sacred cultural site as well as a natural resource that is being desecrated with the addition of more telescopes. The TMT project would add to 13 observatories on the summit.

Sunday’s rally featured music, hula performances and speeches, along with opportunities to sign petitions against the project, which is being financed by TMT Observatory Corp., a Cali­for­nia-based nonprofit with Cali­for­nia and Canadian universities as partners, along with scientists from Japan, China and India.

The company selected Mauna Kea as its preferred site in 2009, and the University of Hawaii, which leases the mountaintop from the state, last year approved a sublease for the project.

The rally "was intended to draw more attention to what’s happening, to allow people to network and build relationships, share and listen and learn," said Imai Winchester, who emceed the event, which he described as a way "to ride the momentum" of activism that has sparked protests across the state, nationally and internationally.

"This isn’t a fad that’s going to go away," said Heeia resident Shelley Mune­oka, a member of MANA — Movement for Aloha No ka Aina.

Nanakuli resident Sean Kau­we­loa signed his name to one of the circulating petitions, which had gathered 500 signatures by 4 p.m. Sunday.

"I want to let TMT know and the state know that it’s not right what they’re doing," he said. "They should listen to the people and stop it. Period. Stop this telescope."

Molokai activist Walter Ritte, who was celebrating his 70th birthday Sunday, said the rally was reminiscent of the Kahoolawe protests that helped set off the Native Hawaiian movement in the 1970s.

"This brings back memories. The last time I was here, I stood up on that bandstand with George Helm and Dr. (Emmett) Aluli to try to save Kahoolawe," Ritte said. "There was only one-quarter of the people that’s here today, and we were successful. We defeated the U.S. military. There’s 10 times more mana (power) here today than there was back then. Mauna Kea is calling all of us together, so we as Hawaiians can harness this mana."

Outside the palace gates, sign wavers lined both sides of South King Street, eliciting honks and cheers of support from passing cars. At one point some of the demonstrators began marching up and down the middle of the busy street, blocking buses and cars.

Sunday marked the 18th day since demonstrators began camping on the mountain after UH declared the area an active construction zone and began hauling heavy equipment to the site.

Thirty-one protesters were arrested April 2 on Mauna Kea for blocking access to workers trying to reach the construction site. The arrests generated outrage within the Hawaiian community and helped galvanize anti-TMT sentiment, triggering protests across the state.

Gov. David Ige has since twice announced a "timeout" of construction activities, the latest in effect until April 20.

Although the TMT project was awarded government approvals following a seven-year environmental review proc­ess that included more than 20 public hearings and a formal contested-case proceeding, legal challenges were still pending when construction was allowed to proceed.

"Because the issue has been taken out of the courts, where people couldn’t really engage, the stakes became higher," said Ilima Long, a political science doctoral student at UH-Manoa. "A halt to construction — now that resonates with us, and now we can be involved."