Meanwhile, PR and public affairs agencies nearby have been reacting to the news by emphasising that their teams are safe and expressing their horror at the events.
Initial media reports were that a car had driven across Westminster Bridge and injured up to five people before striking the railings outside the Houses of Parliament.
Further reports suggested that the driver of the car had then entered the gates of the Palace of Westminster and stabbed a police officer before being shot by armed police as he approached a second officer while still holding a knife. At the time of writing, it's been confirmed that at least one woman has died and a number of others are critically injured.
The Met sent its first tweet out shortly before 3pm.
We are aware of reports of an incident at #Westminster. We will put more information out as soon as we can— Metropolitan Police (@metpoliceuk) March 22, 2017
It then followed up with a second tweet, giving more detail.
We were called at approx 2:40pm to reports of an incident at #Westminster Bridge. Being treated as a firearms incident - police on scene— Metropolitan Police (@metpoliceuk) March 22, 2017
An hour later, the Met issued a further tweet, urging the public to remain calm in the face of the attack.
...before confirming that it was treating the attack as a terorist incident.
Incident in #Westminster: We are treating this as a terrorist incident until we know otherwise— Metropolitan Police (@metpoliceuk) March 22, 2017
Police then called on the public to show restraint if they had captured images or footage of the incident and to pass any material to them for analysis.
Please use common sense and restraint in circulating pictures and videos of those that have been injured during the incident in #Westminster— Metropolitan Police (@metpoliceuk) March 22, 2017
Other agencies, such as TfL and London Ambulance, also used Twitter to alert the public to the unfolding situation.
TfL quickly advised drivers and public transport users to avoid the area.
Westminster station is closed due to a Police investigation— TfL Travel Alerts (@TfLTravelAlerts) March 22, 2017
And London Ambulance also used Twitter to urge the public and media not to stretch its resporces unnecessarily.
As we continue to respond to the incident in #Westminster please only call for an ambulance in a genuine emergency.— London Ambulance (@Ldn_Ambulance) March 22, 2017
As the incident unfolded, the Palace of Westminster and other Parliamentary buildings went into lockdown, with nobody allowed in or out.
A Westminster source, on lockdown with his colleagues, told PRWeek: "We're watching it happen on the news in our office. We're all concerned about our colleagues and what's going on. We're all getting lots of messages from our friends and family and I've been on Facebook to reassure them all. I'm not sure what time we're going to be able to get out."
Meanwhile, the PR industry reacted to news of the incident, with some tweeting to say their teams were safe.
Everyone is safe at Connect. Some of our team were at Parliament but are now safe and away from the buildings.— Andy Sawford (@andy_sawford) March 22, 2017
The FTI PA team is fine - thank you everyone for asking. Terrible times - and sounds like well done to brave police— Alex Deane (@ajcdeane) March 22, 2017
Relieved to say all @intereluk staff are safe but 2 are still in lockdown. Our thoughts are with all those affected. Terrible day.— George McGregor (@GeorgeMcGregor) March 22, 2017
There was also a warm offer to provide shelter for those caught up in the events at Westminster.
If you are caught up in Westminster chaos and need somewhere safe to go, get in touch, we are in 4 millbank and happy to provide shelter— iNHouseComms (@iNHouseComms) March 22, 2017