Commuters have dragged climate change protesters from the roof of a London Underground train.
Extinction Rebellion activists climbed on to trains at Stratford, Canning Town and Shadwell in Thursday's rush hour. Eight protesters have been arrested, British Transport Police (BTP) said.
The Jubilee Line and Docklands Light Railway were temporarily suspended.
Extinction Rebellion later said it would "take stock" of the reaction to the latest action for future protests.
Spokesman Howard Rees said: "Was it the right thing to do? I am not sure.
"I think we will have to have a period of reflection. It is too early to say."
Extinction Rebellion previously said the disruption was "necessary to highlight the emergency".
Hayden Green, a commuter at Canning Town, said he saw the protester "dragged to the floor and kicked repeatedly".
"Police have struggled to deal with the protest in London so the public stepped in and in the heat of the moment it was taken too far," he told the BBC.
"I support their cause but I think how the protests have been carried out has led to more divisions."
In footage shared on social media, a passenger waiting for a train is seen climbing on the carriage to get to one of the protesters.
The activist is grabbed by the knees and dragged down, falling to the platform where he appears to then be kicked and hit by angry commuters on the platform.
Others can be heard shouting and swearing at the protesters.
One shouts: "I have to get to work too - I have to feed my kids."
A second protester was chased along the top of the train carriage by a commuter before being dragged off.
A third Extinction Rebellion activist, who was broadcasting the protest on the group's social media accounts, said he was also attacked and "kicked in the head".
BTP said it was investigating what happened at Canning Town station, adding it was "concerning to see that a number of commuters took matters into their own hands, displaying violent behaviour to detain a protester".
It has appealed for anyone with information, pictures or mobile phone footage of any of the incidents to upload them to its website.
Assistant Chief Constable Sean O'Callaghan said: "It is important that commuters and other rail users allow the police, who are specially trained, to manage these incidents.
"Unfortunately, there is still a risk that Extinction Rebellion will target the rail network during this evening's peak. We will continue to have extra officers on patrol and will work to disrupt any potential criminal action before it happens."
By Justin Rowlatt, BBC chief environment correspondent
Today's Extinction Rebellion action against London's public transport network represents a significant escalation of its strategy of "disruption".
It is one thing to stage a colourful protest in a few roads in Whitehall, quite another to target the Tubes and trains that so many Londoners rely on to get them to work on time.
Many commuters were left scratching their heads this morning, bewildered by an environmental protest that targeted one of the most environmentally-friendly ways to travel.
The tactic has been the subject of much discussion within Extinction Rebellion - a loose affiliation of interested groups and individuals.
A poll among members taken yesterday suggests the vast majority were against any action targeting the London Underground.
Out of 3,800 votes, 72% said they were opposed to any action against Tube trains and 14% were against the idea if people could get blocked underground.
Perhaps not surprisingly, the decision to go ahead has upset many members - as well as commuters - and for good reason.
Tackling climate change will be easier if there is a consensus that action is necessary. The Extinction Rebellion activists behind this action will want to consider whether gluing yourself to a train is really the best way to build that consensus.
At Shadwell station several activists glued themselves to trains, including 83-year-old Phil Kingston.
In April, Extinction Rebellion protesters also glued themselves to a DLR train at Canary Wharf, causing minor delays.
"If XR wants to make an inclusive movement, these tactics on public transport at rush hour won't get them far," Ana Zarraga told the BBC.
The 35-year-old, who was prevented from catching her train at Shadwell, said: "The bankers and the CEOs of the most polluting industries are certainly not travelling on the DLR at 07.00 BST."
Earlier Extinction Rebellion co-founder Clare Farrell defended the Tube action, saying "the public, I don't think, realise quite how serious this situation is".
She added: "Someone has been hurt today. We understand that putting ourselves in these positions is potentially dangerous for us.
"But what else can we do?"
The protester who appeared to try to kick a commuter acted "in self defence in a moment of panic when confronted by a threatening situation," Extinction Rebellion said.
"He acknowledges his accountability for this action," it said.
The group has invited the commuters involved in today's protest "to have a conversation" about what happened.
Mayor of London Sadiq Khan said in a statement: "I strongly condemn the Extinction Rebellion protesters who have targeted the London Underground and DLR this morning.
"This illegal action is extremely dangerous, counterproductive and is causing unacceptable disruption to Londoners who use public transport to get to work."
Train drivers' union Aslef said the Tube and other public transport services were "part of the solution to climate change, not the problem".
Extinction Rebellion should "stick to protesting against those who create the problem - not our industry, members and hard-working commuters", the union added.
A public order ban has been put in place on Extinction Rebellion activities in London since Monday.
At the High Court a judge has refused the request to hear Extinction Rebellion's appeal against the ban early. The group wanted a hearing before the scheduled end of the protest on 19 October.