In 1989 two million people formed a human chain stretching 675km from Tallinn to Riga to Vilnius.
Three countries beside the Baltic Sea – Estonia, Latvia, and Lithuania – were incorporated into the Soviet Union in 1940. Well, in reality they were occupied by Soviet troops following a secret 1939 agreement between Nazi Germany and the Soviet Union, but the official line was that the leadership of those three countries had asked for entry into the new superpower.
This secret pact became public soon after the end of World War II, but the Soviet Union denied that truth for as long as they could maintain (or enforce) their convenient fiction. The independence movements of the three Baltic states saw an interesting legal angle: if they had not joined the union willingly, they could claim that the occupation was always illegal and could be set aside. In other words, automatic independence. While it was technically possible for a member state to leave the union without this loophole, the constitutional laws governing secession didn’t exactly make it easy.
(In fact, the 1977 amendment to the Soviet Constitution made it clear that member states could leave at any time, but it gave no actual mechanism to trigger that departure. In fact, if a member state passed a law to leave the union other parts of the constitution automatically made that law invalid. Clever.)
Anyway, tensions about the Soviet occupation of the Baltic states came to a boil on the 50th anniversary of that secret agreement. The independence movements of Lithuania, Estonia, and Latvia decided that a peaceful display of multi-national unity would be an effective way to promote their cause and put some pressure on Moscow.
The result was the Baltic Way: a chain of people holding hands stretching between the three capital cities. It stretched for around 675km and included an estimated two million people. This is especially impressive if you consider that there were only eight million people total living in those three countries. Naturally, estimates of the total number of participants vary wildly depending on who you believe, but the human chain was enormous by any measure, and influential too. Within two years, all three states were independent.
The Generalist Academy 
No mention of Hands Across America (6m people in 1986)? Which was of course central to Jordan Peele’s “Us.”
I didn’t know about these 2m out of 8m total population! Wow!
In the interests of keeping the post short I skipped over the other notable human chains, but Hands Across America was a big one! (I know it solely from a mention in The Simpsons.)
Other notable human chains: the Taiwanese “228 Hand-in-Hand” chain, the Kerala women’s wall, the Hong Kong Way (held on the 30th anniversary of the Baltic Way), and the record-breaking Manushya Maha Sringhala at the start of this year which had maybe 7 million people.