Parts of Boston were plunged into darkness Tuesday evening when a three-alarm fire broke out in a major electrical transformer causing power outages which disrupted public transit and raised alarm over the potential spread of toxic fumes.

The fire occurred amidst two 115,000-volt transformers located within the densely populated neighborhood and tourist destination of Back Bay, the Boston Globe reported. The fire department subsequently cut power to the transformers, forcing many local businesses to close early with patrons moving out into the streets and eventually evacuated from the area by city buses along with other local residents and business owners.

The public is urged to stay away from this area, read a tweet from the Boston Fire Department at 7:52 p.m. Stay indoors please!

As a plume of dark smoke rose up from the fire, streets were closed, traffic lights and buildings lost power, subway trains were diverted, and a tunnel section of Interstate 90 that runs through the neighborhood was closed off, Reuters reported.

At least 20,000 people lost power in their homes or businesses. Back Bay was not the only neighborhood affected as power outages spread to nearby areas including Chinatown, the Theater District, South End and Kenmore Square.

Despite initial concerns over hazardous fumes coming from the electrical fire, the fire department said there were no signs of a serious threat, the Globe reported. Several people, however, were being treated for smoke inhalation.

The transformer that was on fire is cooled with mineral oil. While not toxic, it is never good to inhale any kind of smoke, read another fire department tweet at 8:50 p.m.

As of Wednesday morning, power was restored to some parts of the Theater District, Chinatown, Bay Village and Clarendon Street in the Back Bay, the Globe reported. Boston Mayor Thomas Menino and an official from NSTAR, the power company that operates the transformers, told the Globe that all power is expected to be restored by Wednesday evening.

The cause of the fire remains unknown, but will ultimately be determined by NSTAR , said Boston Fire Department spokesman Steve MacDonald, the Globe reported. MacDonald said that NSTAR workers have been examining and working on the transformers, one completely destroyed and the other damaged, since midnight.

They are working on getting a good assessment of what happened. There is a large group there trying to identify circuitry and figure out how it can be fixed, MacDonald told the Globe. We believe the transformer shorted itself out from the fire and become engulfed in flames. The second transformer worked to pick up the load from the first transformer and that eventually tripped, affecting power in Chinatown and other substations.

WCVB reporter John Atwater posted a picture on the Twitter of a replacement transformer being delivered to the site.

Crews say the transformer they are moving to replace one that caught fire weighs 100 tons, Atwater tweeted at 8:46 a.m. Emergency operation underway.