Transcript for Attack near UK Houses of Parliament being treated as terrorism: Police

This is an ABC news special. George Stephanopoulos. And we are coming on the air right now because there has been an attack on the British parliament that is being treated as an act of terror that is the scene in London that is a map of London right now the attack. Started around 9:40 AM eastern time on that Westminster bridge you see that right there. According to eyewitnesses. Someone driving a car crossed a bridge plowed into bystanders. Eventually crashed into the gates by the British parliament. And then somebody was able to make it through the gates into the grounds of the British parliament apparently a man with a knife. Who was then director attacked two security guards inside the parliament was shot down at least one person is dead. Right now reports that the gunman is that the assailant is down having been killed. By security grounds guards this morning. The entire parliament are British parliament is on lockdown right now that is the scene on the Westminster bridge heading into the parliament where the attack began. And moments ago the leader of the house of commons leader about the house of commons in the British parliament David Livingston spoke to the members who are on lockdown right now. It seems that it's a police office has been stacked. That's. The alleged assailant. Walls salt by armed police. A and an ambulance is currently attending the scene. To remove the casualties. That. All Olson who were pulls all. Those who have fallen incidents in the vicinity. Of the Palace of Westminster. Popped. I hope colleagues also like to appreciate. But it would be wrong of me here to going to further details until we have confirmation from the police. I'm from the house security authorities about what is going on. I shall endeavor let's do the very best I can both for the dispatch box. And by communicating. Week my opposite numbers in public political parties. To ensure that members architect to web of what is happening but took the movement. The very clear advice from the police. And the threat to security in the house is who we shoot remain. Under suspension and that feed chain bush foods. Remain in lockdown until we receive advice that if he's safe to go back to normal procedures. Our correspondent outs mark where is on the scene in London right now Alex you're across the bridge there from the Westminster castle the parliament. We just heard that the parliament remains on lockdown so police are not confident yet. That this entire incident is contained. Right George they have and actually to find this an act of terror yet but they are assuming. That this is a terrorist incidentally Steve a sense of where we are right here those are the Houses of Parliament inside the Palace of Westminster. That bridge right there is the Westminster bridge you can see traffic has been stopped that entire area has been sealed off cars and buses do remain on the bridge that's where we believe. That the incident started with these suspects in a car going up onto the sidewalk. Mowing down and wounding several people on that sidewalk before continuing along. And driving into parliament square where he again rammed into the gates. Rammed into people as you mentioned he got out and attacked a policeman with a knife she was eventually. Shot and killed now of course this can't sit this country this continent could hardly be at a higher state of alert. In fact the alert level here in Britain is severe meaning that an attack is highly likely this is a very crude. Method of attack that we've seen elsewhere in Europe in the past two years since these terrorist attacks have started of course last year in east. There was that the terrorists who attacked. People L along the K along the beach on Bastille day killing more than eighty people and then just a Christmas. Twelve people killed when another. Terrorists rammed into a Christmas crowd. Killing. A dozen people again the police here have not said that this is a terrorist incident but terror officials here and in the UK have warned that. An attack here in London and beyond is not a question of death but when. It's okay outfit also comes on the anniversary of that attack on the Brussels airport and subway which killed. 35 people that was march 22 2016. Reports from inside the British Poland also say that the British prime minister Teresa may. Was whisked away at the moment of the attack put into a waiting car and taken away. From the grounds of parliament as we said this began this tech again on that bridge that bridge that Westminster bridge heading towards the parliament. And a photographer lot of slot Sikorski was an eyewitness today and spoke to eyewitness is everyone here what he had to say. It is. I think from mom questioned foot square to disrupt and I didn't even competent. What I thought all of a yeah I'm just a commission. And I looked. Through the window of the action not someone who Dalton. Almost feel great distress from the Bucs are second vote it down. And I started building and that might also relocate pulled down one of them BP rep refused. So what I coordinate them and holding it to leave fighting people Syracuse and. When looking at the video that you shot just. Moments ago would could you describe what reaction there was what with a police were already that what what what happened. Yeah another look this disclosed. I started sewer you infections often they're called polyps and people rushed forward to help the victims. So you. You saw the up to multiple what looks like a car driving into several people. The early mowing down. Ice bowl five C fight people down. And that won't happen. And it and then by taxi move a way he. And the taxi drive it's called the police are. And then move then we started hearing that's how the ambulances. It's on the bridge their honest plus of course human that's who did he say at least five people. Mowed down by that vehicle of course this is put her own Department of Homeland Security on high alert as well it's gonna Pierre Thomas with the latest on that. George senior FBI officials and Homeland Security officials as you say are where they're tracking this incident closely. Early got to say have is what you've been reporting to. Let's separate crime scenes one involving an officer being stabbed at the parliament. Another involving people being mowed down five vehicle. Law enforcement sources telling us that and investigations under way to see if they are tied. People here are considering this a terrorist incident even though it has not been officially described as such. Law enforcement officials deeply concerned given bridges the close working relationship that we have of the British government. And we are also being told now the police here in New York City are increasing security several high profile British targets. Here in New York City Cecilia Vega at the White House the president already has been briefed by as national security advisor. I HR McMaster has national security writer George this we believe this briefing happens sometime around 11:30 this morning the president is here at the White House in meetings mostly. About health care today we haven't heard much more. We have reached out to senior White House officials to find out exactly what the president knows and what the reaction from the White House is in will be. The president did make a brief comment about this in front of cameras today they were in the room for. These meetings on health care the president apologize for being late. Two women in the room and that he said he was being briefed in and was apologize that he was dealing with. That situation there in London but Georgia's you know day 62 of this presidency in this would be. Perhaps one of the first large scale terror attacks if that ends up being what this is that that president trump what have to respond to here. OK Cecilia Vega and our chief global affairs anchor Martha Raddatz also with us by phone. Right now and Martha this of course comes on the heels of a 24 hours after the United States. And Great Britain both announced a ban. And certain electronics coming from direct flights from mid for majority Muslim countries. He certainly did and that's what local authorities or argue which authorities or shall we should not here but laptops from those majority. Muslim countries being banned from certain airports aren't a slight into the United States this is an entirely different kind of attack. If it turns out to be a terrorist attack. A talent that it very crude. Apparently so far there is only one person involved that we know of but whether there are others is what they're drastic right now that's why that Harriet aren't locked down and they continue. To bring an ambulances and other parties. Again the British parliament on lockdown thank you Martha Raddatz the British prime minister. Has been taken to secure location a terrorist attack being treated as a terrorist attack. In London this morning about five hours ago at least five people scene injured so far that number could climb as well. It appears according to police reports that the assailant has been killed inside the grounds of parliament organist on top of this all day long when back. With special reports as we get more information hundrer Stephanopoulos in New York. This has been especially from needy.

This transcript has been automatically generated and may not be 100% accurate.