Multiple gunmen, reportedly armed with machine guns, opened fire at two beach resorts in Tunisia.
At least 27 people have been killed.
One wrote on Twitter: "Actual terrorist attack on the beach next to my hotel. Gunshots and grenades going off all over the place.
"Sat in the pool and hear a noise like a gun, then an explosion then multiple shots and staff getting us back to the rooms."
Actual terrorist attack on the beach next to my hotel. Gunshots and grenades going off all over the place
Elizabeth O'Brien, from Dublin, said she grabbed her two sons and they ran for their lives when they heard gunfire at one of the hotels.
"We were on the beach, my sons were in the sea and I just got out of the sea," she said.
"It was about 12 o'clock and I just looked up about 500 metres from me and I saw a (hot air) balloon collapse down, then rapid firing, then I saw two of the people who were going to go up in the balloon start to run towards me - because I thought it was fireworks," she told RTE Radio.
"So, I thought 'oh my God, it sounds like gunfire', so I just ran to the sea to my children and grabbed our things and as I was running towards the hotel, the waiters and the security on the beach started saying 'run, run run!' and we just ran to our room, which is like a little bungalow.
"So we are actually trapped in our room."
Tourists are now gathered in hotel reception areas and hiding in rooms as the situation develops.
According to security sources cited by Reuters news agency, one gunman has been killed.
In a statement, Thomas Cook said: "Thomas Cook has been advised of an incident that occurred earlier today in Sousse, Tunisia.
"At this time, details are not clear as to which property(ies) have been affected, with conflicting news reports.
"We are currently gathering information and will provide an update as soon as possible. Our teams on the ground are offering every support to our customers and their families in the area.
"We will continue to monitor the situation, working closely with the FCO and local authorities."
The Foreign Office said people in the UK concerned about relatives in Tunisia should call them on 0207 208 1500.