When Martha Poehler walked from her Cambridge home to Wednesday’s memorial service for slain MIT police officer Sean Collier, she knew she wouldn’t be able to get inside Briggs Field. The service was reserved for law enforcement officers and those affiliated with the university.

But she needed to be there, just the same. So she listened to the service, which drew thousands, under a tree just beyond Briggs Field.

It was, she said, “the least I could do.”

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“He sounded like an incredible young man,” said Poehler, 66. “I would have loved to know him.”

Dozens of Cambridge and Somerville residents gathered just outside the service, bound by a shared sense of obligation to honor Collier’s memory.

“I just wanted to pay my respects,” said Mickey Barry, 64, a technician at Draper Laboratory in Kendall Square whose good friend knew Collier well.

Maurice Martin, a neighbor of Collier’s in Somerville, wasn’t able to get into the service, but wound up watching it on TV. But to be near, to see the thousands of police officers who attended, was worth it, he said.

“He was a hell of a good guy,” he said. “It meant something for me to be here.”