Protesters affiliated with the activist group Black Lives Matter carried out their most audacious and disruptive demonstration yet in the Boston area Thursday, blocking the largest highway into the city, snarling rush-hour traffic, and stopping an ambulance in its tracks.

In a pair of coordinated surprise actions that angered commuters, the protesters formed human barricades on Interstate 93 north of the city in Medford, and south, in Milton. Some put barrels filled with concrete on the highway and chained themselves to them. Twenty-nine protesters were arrested.

The well-organized protests were carried out on Martin Luther King’s birthday by activists calling attention to the deaths of unarmed black men at the hands of police in Missouri and New York.