HONOLULU — More than 2,500 astronomers from around the world are in Hawaii for a conference at a time when plans to install telescopes on two mountain summits have led to a standoff between scientists and cultural and environmental activists.

The International Astronomical Union’s general assembly, which starts Monday, was planned years in advance but is happening amid protests because the mountains are held sacred by Native Hawaiians.

Some demonstrators on Maui and the Big Island were arrested Friday as scientists from more than 75 countries began arriving in Hawaii.

The protests gained momentum in April when workers tried to start construction on the Big Island’s Mauna Kea of what would be one of the world’s largest telescopes. Protests also have spread to a solar telescope being built on Maui as Native Hawaiians and others demonstrate over issues such as development, sovereignty, and religious rights.


Twenty people were taken into custody on Maui while trying to block tractor-trailers headed to the project under construction on the summit of Haleakala. Demonstrators lay on the ground and connected themselves with plastic pipes and chains, Maui police said.

On the Big Island, seven others were arrested for allegedly defying a rule created to stop people from camping on Mauna Kea.

Conference organizers and police are bracing for peaceful demonstrations near the convention center in Honolulu.

Kealoha Pisciotta, one of the protest leaders, said demonstrations are expected to include mostly people holding signs. She said she hopes attendees take time to learn why protesters oppose the telescope on Mauna Kea.

Protesters say they’re not against astronomy but believe building on those mountains represents desecration.

‘‘As the IAU and our conference are not affiliated with the construction of telescopes, the focus of the general assembly is to offer scientific presentations, policy discussions and meetings to the conference attendees,’’ Piero Benvenuti, the organizations’ assistant general secretary, said in an e-mail.


Convention plans previously offered an optional excursion to Mauna Kea, but that was canceled after the mountain’s access road was closed indefinitely, Benvenuti said.

In advertising the excursion on the conference’s website, organizers described Hawaii’s history with astronomy, saying Polynesian sailors navigated widely separated Pacific islands ‘‘primarily using their deep knowledge of the starry sky.’’

Convention organizers are disappointed that visiting astronomers won’t be able to visit what’s considered one of the world’s foremost sites for astronomy, Benvenuti wrote.

However, ‘‘we deeply respect the views of all parties involved, and it is our sincere hope that moving forward, there will be an open, productive dialogue among all parties involved, creating a shared, long-term vision for Mauna Kea,’’ he said.

The million-year-old mountain is a long-dormant volcano formed by magma oozing up from the Earth’s interior. It looms nearly 14,000 feet above the surface of the ocean and more than six miles above the sea floor, making it the world’s tallest mountain from base to peak.

Native Hawaiians consider its summit the dwelling place of gods.

But Hawaiians of all ethnicities are debating whether benefits brought by the telescope project — jobs, tourism, a $1 million-per-year lease, better views of the universe’s outer reaches and distant path than any we’ve seen before — outweigh the potential costs to their cultural heritage.

The telescope, which will see as much as 100 times farther and more clearly than anything that has come before, allowing astronomers to gaze toward the distant reaches of the universe, is backed by institutions in the United States, Canada, China, India, and Japan, as well as the Gordon and Betty Moore Foundation in San Francisco.


Its opponents are mostly local leaders who have thrown lawsuits, petitions, and sometimes their own bodies in the path of construction.

Work on the Thirty Meter Telescope — named for the diameter of its enormous, light-gathering mirror — has been disrupted since workers broke ground on the project last fall.

The Hawaii Supreme Court is due to hear oral arguments this month in a case challenging the TMT’s permit.

In early April, hundreds of protesters gathered at the mountain’s visitors center and summit in an effort to blockade work crews. More than 30 were arrested, triggering more demonstrations across the state and pushing Governor David Ige, a Democrat, to postpone construction briefly.

Ige has at times seemed both sympathetic to and frustrated by the protesters. After the initial protests in April, he acknowledged that the state and the scientists who work on the mountaintop ‘‘have not done right by a very special place.’’