After numerous attacks including from Aru and Porte, Froome dropped the other GC rivals, with Nairo Quintana (Movistar) and Aru unable to stay with the three-time Tour de France victor. Porte was in a group of seven riders, including race leader Chris Froome, who were descending from the last of seven climbs on Sunday.
Porte was taken by ambulance to hospital as Froome raced to pull in Romain Bardet (AG2R) at the front, which he did with 2 kilometres to go.
Thomas had crashed on another descent much earlier in the stage and quit the Tour with a fractured collarbone.
Later on the slopes of the fearfully steep Mont du Chat, Martin - who was sitting in fourth place at the start of the day - and fifth-placed Porte came to grief, when the latter fell and brought down Martin.
Earlier, Geraint Thomas of Team Sky also crashed out of the race. "I want to thank the other riders, who did not attack".
Porte had been having an exceptional day on the bike, riding in the yellow jersey group chasing the last remaining breakaway rider, Warren Barguil (Sunweb), when he appeared to misjudge a bend.
The two-times Tour victor Alberto Contador also hit the deck as he fell on the ascent of the Grand Colombier, but the Spaniard was quickly back on his way with a push from spectators.
Astana's Fabio Aru moves to second overall and was involved in one of the day's main talking points when he attacked Froome on the ascent of the Mont du Chat just as the race leader raised his arm to signal a mechanical problem.