Byzantine emperor Alexius I Comnenus sent a request to the west for help against the Seljuk Turks. The message was received by Pope Urban II at the Council of Piacenza; later that year, in November, Urban called the Council of Clermont to discuss the matter further.
The Council lasted from November 18 to November 28, and was attended by about 300 clerics from throughout France. Urban discussed Cluniac reforms of the Church, and also extended the excommunication of Philip I of France for his adultery. On November 27, Urban spoke for the first time about the problems in the east.
There are six main sources of information about this portion of the council: Fulcher of Chartres, Robert the Monk, Baldric of Dol, and Guibert de Nogent, who were apparently present at the council; also the Gesta Francorum or The Deeds of the Franks, and a letter written by Urban himself in December of 1095.
According to Fulcher, Urban addressed various abuses of the church such as simony and the lack of adherence to the Peace of God. He then asked western Christians, poor and rich, to come to the aid of the Greeks in the east, because "God wills it." Fulcher records that Urban promised remission of sins for those who went to the east, although he probably did not mean what later came to be called indulgences.
Robert the Monk, writing about 20 years after the council, recorded that Urban's emphasis was on reconquering the Holy Land rather than aiding the Greeks. According to Robert, Urban listed various gruesome offenses of the Muslims, but did not mention indulgences or advise anyone but knights to go. Robert's version was essentially an extended version of the speech recorded in the anonymous Gesta Francorum, written around 1100.
Baldrick also based his account on the Gesta Francorum, and wrote around the same time; he too focused on the offenses of the Muslims and the reconquest of the Holy Land. Like Fulcher he also recorded that Urban deplored the violence of the Christian Knights, and wanted to put it to better use against the Muslims.
Guibert of Nogent also recorded that Urban's emphasis was reconquest of the Holy Land, but not ncessarily to help the Greeks or other Christians there; Urban's speech, in Nogent's version, said that the Holy Land must be in Christian possession so that prophecies about the end of the world could be fulfilled.
Urban's own letter does not mention Jerusalem at all; he only calls for help for the Eastern Churches, and appoints Adhemar of Le Puy to lead the Crusade.
On the last day of the council, a general call was sent out to the knights and nobles of France. Urban himself spent a few months preaching the Crusade in France, during which time the focus presumably turned from helping Alexius to taking Jerusalem; the general population, upon hearing about the Council, probably understood this to be the point of the Crusade in the first place.