After these things had been done, the officials approached me and said, "The people of Israel, the priests, and the Levites have not separated themselves from the peoples of the lands with their abominations, from the Canaanites, the Hittites, the Perizzites, the Jebusites, the Ammonites, the Moabites, the Egyptians, and the Amorites. For they have taken some of their daughters as wives for themselves and for their sons. Thus the holy seed has mixed itself with the peoples of the lands, and in this faithlessness the officials and leaders have led the way." When I heard this, I tore my garment and my mantle, and pulled hair from my head and beard, and sat appalled. Then all who trembled at the words of the God of Israel, because of the faithlessness of the returned exiles, gathered around me while I sat appalled until the evening sacrifice. (NRSV Ezra 9:1-4)
Now when these things were done, the princes came to me, saying, The people of Israel, and the priests, and the Levites, have not separated themselves from the people of the lands, doing according to their abominations, even of the Canaanites, the Hittites, the Perizzites, the Jebusites, the Ammonites, the Moabites, the Egyptians, and the Amorites. For they have taken of their daughters for themselves, and for their sons: so that the holy seed have mingled themselves with the people of those lands: yea, the hand of the princes and rulers hath been chief in this trespass. And when I heard this thing, I rent my garment and my mantle, and plucked off the hair of my head and of my beard, and sat down astonished. Then were assembled unto me every one that trembled at the words of the God of Israel, because of the transgression of those that had been carried away; and I sat astonished until the evening sacrifice. (KJV Ezra 9:1-4)
This idea of the "holy seed" is a new one introduced by Ezra: the only real Israelite is a person with two Jewish parents, no exceptions. You can't convert, you can't as a foreigner decide to follow God's laws. You either are or are not kosher by birth and nothing can change that. This wasn't an idea that was adopted by everybody. People willing to accept the God of Israel and follow the commandments were typically welcomed into the community. Even King David, God's anointed, was the grandson of Ruth, a Moabite convert to Judaism. Ezra was upping the ante here in an effort to maintain the ethnic purity of the community (source).