According to Halacha, a bride and groom must undergo two stages to become married: Kiddushin and Nisu'in. Kiddushin (betrothal) is effectuated when the groom gives the bride something of value – generally a ring – and declares, "Harei At Mekudeshet Li" ("You are hereby betrothed to me"). The process of Nisu'in, however, is subject to controversy. According to the view of the Mordechi (Rabbi Mordechai Ashkenazi, Germany, 1240-1298), Nisu'in is achieved when the bride and groom stand together under a Tallit and the seven special blessings (the "Sheva Berachot") are recited. We, however, follow the view of the Shulchan Aruch, who, based on the position of the Rambam (North Africa, 1135-1204), rules that the marriage process is completed by "Yihud," the bride and groom's seclusion in a private room.
When should the "Yihud" of the bride and groom take place?
The prevalent custom among Ashkenazim is to conduct the "Yihud" immediately following the Hupa ceremony. Sepharadim, however, have, for many generations, followed the practice of conducting the Yihud only after the wedding. If the marriage is finalized immediately after the Hupa ceremony, the bride would be required – like all married women – to cover her hair already at that point. The bride and groom must therefore ensure not to complete the Nisu'in process after the Hupa, and they should be secluded only after the wedding. If they wish, they can go into an unlocked room after the Hupa to eat and exchange gifts for a few moments, but the formal Yihud should not take place until after the wedding. This is the position taken by many Halachic works, including the Shiyurei Kenesset Ha'gedola, Mas'at Binyamin, Sha'ar Ha'melech and Zivchei Tzedek. Chacham Ovadia Yosef expresses particular vehemence in this regard, insisting that Sepharadim not break from this time-honored tradition to conduct the "Yihud" only after the wedding.
One question, however, arises concerning this practice. How can we allow reciting the Sheva Berachot – the blessings recited over the Nisu'in – under the Hupa, if the Nisu'in will take place only several hours later, after the wedding?
Chacham Ovadia addresses this question in his work Yabia Omer, and explains that we follow the view of the Ran (Rabbenu Nisim of Gerona, Spain, 1290-1380), who classified the Sheva Berachot as "Birkot Ha'shevach," blessing of praise to God, rather than blessings recited over the Mitzva of Nisu'in. When one recites a Beracha over the performance of a Mitzva, then indeed the recitation must occur immediately preceding the Mitzva's performance. This is not the case, however, with regard to Berachot recited as praise to God over a joyous event, and thus we may recite the Sheva Berachot under the Hupa despite the fact that the Nisu'in will take place only several hours later.
Must witnesses be present when the bride and groom enter a private room for "Yihud"?
Although the Ashkenazim generally require the presence of witnesses for "Yihud," our custom is to conduct "Yihud" without witnesses. As noted by the Aruch Ha'shulchan (work of Halacha by Rabbi Yechiel Michel Epstein, Byelorussia, 1829-1908), everybody knows that a bride and groom get married in order to live together privately, and there is therefore no need for specifically-designated witnesses to their initial seclusion.
Summary: Sepharadim should follow their time-honored tradition that the "Yihud" (seclusion) of a bride and groom takes place only after the wedding, and not immediately after the Hupa ceremony. If the bride and groom wish, they may spend some time together in an unlocked room after the wedding ceremony, but the formal "Yihud" should take place only after the wedding. Witnesses are not required for the bride and groom's "Yihud."