Hundreds of fish have been killed in a pollution incident in a river in Wales.
The source of the pollution in the Afon Llynfi, a tributary to the River Wye, is being investigated by Natural Resources Wales (NRW).
NRW said about 200 bullheads had been found dead at the site, near Pontithel in Powys.
Emily Tilling, who lives near the river said the number of dead fish, which included brown trout and grayling, was many hundreds more.
She raised the alarm after noticing an "overpowering chemical smell" during a dog walk on Friday evening.
She said: "I called it in to NRW from the riverbank straight away, and when I went back later on there were crayfish crawling up the bank to escape the water, and fish lying on the rocks gasping for breath.
"There were eels struggling by the shoreline. I've never seen anything like it.
"The river had been running clear that afternoon, but it had turned a dirty grey-green colour and there was an overpowering chemical smell in the air, slightly chlorinated."
Another witness, David Brunsdon, from Caerphilly, said the scene was "heartbreaking".
"As we walked across the field, the smell hit us. It smelt like something dead, rotting. It was a horrible, musky smell," he said.
"There were hundreds upon hundreds of dead fish - from tiny fry, to Grayling, to a sizeable, nine-inch trout. I think there were between 500-1500 dead fish."
NRW said: "We are currently at the scene of an ongoing pollution incident at Pontithel, near Greater Porthamel.
"It has resulted in around 200 dead bullheads in a seven metre stretch of the Llynfi river. The investigation into the source of the pollution is ongoing."