San Francisco Airport has been evacuated and police are on scene amid reports of an unattended bag and a family bypassing security by entering through the exit.
Angry passengers report police swarming the area with planes being moved and people being forced away from the building.
San Francisco International Airport has only said there was an "unattended item" in the Terminal 2 food court and has been accused of handling the situation "ridiculously".
One person asked "are you waiting for someone to lose it and start a stampede?"
San Francisco International Airport said in a statement on Twitter: "#SFOAlert - SFPD responding to Terminal 2 food court area to clear an unattended item. Food court and D gate areas being cleared out of abundance of caution."
Traveller Lisa Alpert took to Twitter to alert her fellow travellers.
She said: "Everyone around the Terminal 2, D gate food area were evacuated then asked not to move as security assessed the situation. Security lines were also halted. "
She added that the Terminal was evacuated, "but if you were exiting the airport or going to another wing, you were allowed to. We are in Terminal 2, C wing and they halted boarding to one flight, but are allowing boarding on others. Unclear what is going on."
Emergency vehicles were reported arriving at Terminal 2, with crews evacuating, according to Erik Charlton, who tweeted: "SFO Terminal 2 - crews evacuating and emergency vehicles arriving."
Other travellers took to Twitter to find out what was going on.
On Twitter, @DLoutsis said: "What’s going on @flySFO …. Captain just told us law enforcement is telling our plane to move as we were waiting to go to our gate, and we are now moving farther away from the building. Do to a law enforcement issue."
@allie_oldham replied: "Bag left in the food court area unattended. Terminal is being evacuated as we speak."
@hhekas1 tweeted: "Something happening at SFO right now, made all of Gate D evacuate and the police have sectioned off an area by the gate."
Another Twitter user sais that "everyone is being evacuated from terminal 2 and now everyone is trying to get into T3."
@FoodWineMaven claimed: "TSA agents waiting for cops to apprehend family that skipped security by entering terminal through exit."
Images posted online show massive crowds building up as worried travellers are moved on and wait to hear what is happening.
@carolynlight posted a video of crowds waiting and said: "Evacuation Terminal 2 at SFO. Fire truck and ambulance."
San Francisco International Airport has not revealed any more than their initial tweet, on their official @flySFO account, had detailed.
One angry passenger wrote: "@flySFO any updates? I’m sitting on a plane that arrived and is waiting for a gate with lots of folks nervous about missing connections."
@adambcollins1 wrote: "It would be really great to the 1000’s of passengers if you’d make some sort of announcement, post something on a board or ANYTHING other than letting people wander around in herds with no info about their flights. I was told my flight would be held. It left. My new flt T2 also."
Kelly Winn-Boaitey said: "So ridiculous how this is being handled. We are crowded in the are just outside Terminal D and Ms sassy pants TSA lady needs some customer service. Are you waiting for someone to lose it and start a stampede?"
Another replied: "Updates needed for folks that have flights that still are reporting as'on time,'" and due to take off within the hour."
Reena Rahal wrote angrily: "There are no ticket agents at Air Canada, what is going on and why isn’t anyone giving us updates on our flights????"
We'll be bringing you the very latest updates, pictures and video on this breaking news story.
