Footage posted on social media appears to show fireworks being shot at two Just Stop Oil activists who scaled a bridge.
The video seems to capture the explosives being aimed at the climate change protesters as they sit atop the 275ft masts on Dartford Crossing, also known as the Queen Elizabeth II bridge.
Red and orange sparks can be seen lighting up the sky near the top of the bridge, with people heard laughing as the fireworks boom.
Kent Police said they were making enquiries into the circumstances of the post.
The footage is captioned with two laughing face emojis and seems to have been filmed on the Kent side of the bridge near Greenhithe.
The protesters scaled the masts at 3.50am on Monday morning, forcing police to shut the bridge.
They have now been in place on the support cables of the bridge, hanging in hammocks, for more than 30hrs, as the Just Stop Oil protests - set to last all month - reach their 18th consecutive day.
A Kent Police spokesperson said: "Kent Police is aware of a video circulating online which appears to show fireworks being set off in Greenhithe near to the QEII bridge, Dartford.
"Officers are making enquiries to ascertain the circumstances surrounding the video and to identify any offences which may have been committed."
The environmental campaign group want the British government to end all new gas and oil licences in an effort to stop their detrimental effect on the planet.
Traffic heading from Essex to Kent were forced to divert using one of the two Dartford tunnels which normally bring M25 traffic in the opposite direction.
It caused thousands of commuters endured severe delays adding hours to their journeys.
Frustrated motorists experienced further traffic today as the protest continued.
Queues on the A2 heading to the M25 stretched for more three-and-a half-miles from both Northfleet, Kent.
Tailbacks also affected the Blackwall tunnel in south east London with further three and a half mile queues as drivers tried to find alternative routes.
One of the two bridge protesters, Trowland Morgan, 39, a civil engineer from London said today: “We just survived the first night up here. I was pretty cold, didn’t sleep much but I’m OK.
“We’re up here until the Government makes a meaningful statement to cancel all new licences and consents for oil and gas extraction.
"Oil and gas are killing us now and it’s driving millions into poverty this winter.
“We have to have an emergency transition to renewable energy, quite a stable, affordable energy.
"We’ve got to pay for that transition using the profits of the oil and the gas industry and the rich.
“Now is the moment for civil resistance. Come and do what you can at this pivotal moment in history.”
Essex Police said on Monday night: "Our officers are continuing to work with partners, including National Highways and Thurrock Council, to safely resolve the situation on the Dartford Crossing where two people are at height.
"The QEII Bridge remains closed while we work alongside our partners.
"During the busy Monday evening rush hour period a contraflow stayed in place through the tunnel to ensure traffic could move from one side of the river to the other. We expect the bridge to remain closed this evening and into tomorrow morning’s rush hour.
"We would advise motorists to avoid the area and plan an alternative route if they possibly can.
Chief Superintendent Simon Anslow said: “We understand how frustrating this situation is for people wanting to use the bridge and go about their business. Let me assure you we are doing all we can to resolve this situation quickly and safely.
“It is a complex situation due to the height the people on the bridge are at.
“Our colleagues at National Highways, who have responsibility for the bridge, have made an assessment that it is not currently safe to re-open the road.”
Chief Supt Anslow added: “We will take action against anyone who puts life in danger or thinks they can selfishly stop others getting to and from work or just going about their daily lives. The impact is far reaching, especially for the local community.
“I would again like to re-iterate my thanks to the public for their patience and understanding.
"I assure them that we continue to work with our partners to bring this dangerous and irresponsible disruption to an end.
“Our priority is to keep people safe and keep Essex moving.”
Meanwhile, from around 7.45am this morning Just Stop Oil protesters sat blocking Talgarth Road, near the Hammersmith flyover. Some glued themselves to the street while others locked on to each other.
Police said they were "rapidly on scene" and arrested 26 activists.
"Specialist officers attended and unglued those who had stuck themselves to the road or each other," the Met Police said in a statement.
"All were arrested on suspicion of wilful obstruction of the highway and taken into custody at a south London police station."
Police had reopened the road in both directions by about 9.30am.
