Activists renewed their demand Wednesday that the Baker administration revoke an air-quality permit issued for a proposed natural gas compressor station, gathering outside the headquarters of state environmental regulators to complement an appeals hearing underway challenging the permit's validity.

While the first day of a three-day hearing unfolded inside the Department of Environmental Protection, several dozen members of the Mothers Out Front and other advocacy groups took to the streets to warn that the station would bring environmental and public-health harm to environmental justice communities in the Weymouth area.

"Emissions from this dangerous and dirty fossil fuel infrastructure will threaten the health and safety of the children of the Fore River Basin and beyond," said Anne Goodwin, one of the leaders of Mothers Out Front's Massachusetts chapter. "The northeast pipeline expansion this would be a part of will contribute to climate change at a time when we urgently need to stop using fossil fuels."

Energy company Enbridge is seeking to place its station in Weymouth to help connect its natural-gas pipeline infrastructure from New Jersey up to Beverly. After producing a health impact statement, the Baker administration in January approved an air quality permit, one of the project's key requirements, despite widespread opposition from elected officials and others. Citizen groups joined the city of Quincy and towns of Weymouth, Braintree and Hingham in appealing the decision in February.

DEP officials began a three-day hearing on the appeal Wednesday, where dozens of community members, experts and officials are expected to speak. Mothers Out Front is not a party in the appeal, but volunteer leader Carol Chamberlain said members decided to hold their press conference with other advocacy groups to help raise attention around the issue and put pressure on Gov. Charlie Baker to intervene and revoke the permit.

"We're hoping it will be synergistic by proceeding on different fronts," Chamberlain said.

Goodwin said activists had staked out Baker's arrival to Logan International Airport this morning ahead of his trip to Washington to hand-deliver "postcards" urging him to take action. They also visited the governor's State House office after the rally to leave additional postcards.

A report released earlier this week from the Greater Boston Physicians for Social Responsibility cautioned that the station would be placed in a densely populated area where access for first responders is challenging.

"Governor Baker, do the right thing," said Amy Tai, another speaker with Mothers Out Front. "Do the moral thing. Listen to the scientists, doctors, elected leaders, boards of health, first responders and residents of the Commonwealth. Rescind this permit. Stop this project."

Baker in the past has said he had "no choice" but to approve the project because of federal rules and the result of the state's review.

On Monday, asked about the latest concerns about the project's health and safety impacts, Baker told reporters, "We've been hearing all kinds of points of view on this issue now for over two years." He noted the project proponents had sued the state over the length and "broadness" of its review, resulting in a settlement requiring a decision soon.

"There's an open public process going on right now," Baker said. "If people have issues and concerns, they should absolutely file those, include them as part of the record, and they'll get a serious review. But remember, this is what I would describe as a process where the state has a role as sort of an administrator of federal rules and federal laws and federal requirements, and in addition to raising these issues, which are really important, with us, I would urge people to raise them with FERC and with their congressmen and their senators."

Although several dozen critics of the compressor station filled the audience wearing stickers criticizing the proposal, the first three-plus hours of the hearing featured mostly technical discussions about specific details cited in written reports.

All of the roughly two dozen witnesses set to be called have already submitted written testimony, and the hearing will focus almost exclusively on various parties in the case performing cross examination to follow up on components of those pre-submitted comments, according to a DEP spokesman.

According to the appeal, the compressor will be driven by a combustion turbine and exhaust gases will be emitted through a 60-foot stack at an exit velocity of 28 feet per second and temperature of 999 degrees.

The petitioners behind the appeal allege, among other things, that the agency's air quality permit approval is based on flawed analysis and modeling and inaccurate and inadequate data.

Jane Rothchild, DEP's presiding officer in the hearing, wrote in appeal documents that she did not believe settlement of the appeal was likely. She also wrote that claims regarding facility siting, safety concerns, environmental justice, and reasonably foreseeable climate change impacts may not state claims for which relief can be granted in this proceeding" and would be best addressed by a motion to dismiss.

"The Department and the Applicant indicated that they intend to file such a motion, to which the Petitioners may respond," Rothchild wrote. "If the Motion to Dismiss is denied as to any of the claims challenged, then those issues will be among those resolved after the hearing."

Rothchild wrote that the scope of the appeal is limited to issued regulated by DEP's air permitting program.

Executive Office of Energy and Environmental Affairs spokesman Peter Lorenz said in a statement that the state's review ahead of awarding the permit was exhaustive and sufficient.

"While the Federal Energy Regulatory Commission has primary jurisdiction over the siting of interstate pipeline projects, the Commonwealth completed a comprehensive and science-based evaluation of air quality and health impacts in advance of approving an air quality permit for a proposed natural gas compressor station in Weymouth, Massachusetts," he said. "Massachusetts continues to conduct other required reviews under the federal Natural Gas Act, and will continue to prioritize the Commonwealth’s public safety and the proper environmental protections in reviewing all proposed projects."

Michael P. Norton contributed reporting.