There are two organizations involved, the US Anti Doping Agency (USADA) and the Union Cycliste Internationale (UCI - it's french).
The USADA ruled that he had violated anti-doping regulations based on the testimony of his teammates. Armstrong declined to follow the USADA appeals process because he said that the process was stacked against him. The case was based entirely on witness testimony (no physical evidence) and the witnesses were all allegedly offered plea deals to testify (they each received a six-month suspension versus Armstrong's lifetime ban and forfeiture of everything he won in exchange for admitting that they had cheated and had done it with Lance Armstrong), so they allegedly had a motive to say whatever the USADA told them.
The decision was then left to the UCI regarding what to do about his titles, and they declined to take action until earlier this month when the USADA released all of their case information.
Ultimately, it looks like everybody in cycling was cheating for 10 or more years in a row (they can't find non-cheaters to award the vacated titles to) and they decided to make a statement by going after the biggest winner and taking him down.
If by "proven" you mean physical evidence that he took performance enhancing drugs or did other things like this, no. Apparently many teammates, spouses of teammates, doctors, and others testified that they took steps to help cheat and avoid detection.
More from ESPN if you are interested.