Trump, who overnight prematurely declared himself the victor of Tuesday's election, has spent months denouncing mail-in ballots, making unsubstantiated claims that they are liable to fraud.
Several sheriff's deputies blocked the entrance to the building.
Chris Michael, 40, from Gilbert, Arizona, said he came to make sure all votes are counted.
On Wednesday, a group administrator posted an "Urgent call to action in Detroit", in Stand Up Michigan to Unlock Michigan encouraging any users stationed outside the TCF Centre to "just be a presence" and to "pray, take photos, take videos".
Trump's allies chanted, "Stop the count!" inside TCF Center, where ballots were being handled.
The AP's projection comes hours after the Trump campaign filed a suit to halt counting of ballots in MI after they argued the campaign hadn't been given "meaningful access" to observe the opening of ballots and the counting process.
Two top county officials - one a Democrat, the other a Republican - issued a statement expressing concern about how misinformation had spread about the integrity of the election process.
Joe Biden Sweeps Dixville Notch; Last Candidate to Do So Lost Presidency
Hillary Clinton won Dixville Notch in 2016, and went on to win New Hampshire's four Electoral College votes. However, this year, Hart's Location opted to discontinue the tradition due to the COVID-19 pandemic .
A woman gives the thumbs down at a Central Counting Board vote-counting center in Detroit. "MI will always continue to protect the rights of all voters to have their ballots counted". "An accurate vote takes time".
In Portland, anti-Trump protesters clashed with police.
Republicans are hoping to challenge the vote and claimed they were unfairly being kept outside where the absentee ballots are being counted.
"It's important to trust the process, and the system that has ensured free and fair elections in this country through the decades, even in times of great crisis", Brown said in a statement.
MI has been on edge for months over fears of political violence.
Scattered protests broke after voting ended last night, stretching from Washington DC to Seattle, but there was no widespread unrest or significant violence.