Part of the group that met to discuss the storming of the American embassy in Iran advocated storming the Soviet embassy. What if the group had succeeded in having the Soviet embassy selected as the target to be stormed, or broken away to storm it on their own?
The Soviet Union had a very hardline reaction to groups interfering with embassy staff and it had disputes with Iran, so special operations and arming of groups favorable towards its interest would be a likely response at the very least.
I was referring more to the infamously brutal reprisals that followed the 1986 kidnapping of Soviet diplomats in Lebanon.
I can see an invasion. The Islamic Republic will be toppled with both the United States and Soviet Union pissed with them.