How does Shakespeare present Othello to contradict Iago's and Roderigo's descriptions of him at the beginning of the play?

The play begins with the disgruntled Iago and Roderigo who envy Othello for his position and marriage. With this beginning Shakespeare's presentation of Othello undercuts Iago's and Roderigo's description of the general and shows the reader that these characters' descriptions are motivated by jealousy and ulterior motives. In Act I:i, Iago bemoans his lack of advancement under Othello to Roderigo, who aches for the love of Desdemona. When Iago tells Roderigo of Othello and Desdemona's marriage, they decide to "call up her father / . . . poison his delight / . . . incense her kinsmen" (I.i. 67-69) in an effort to undermine Othello and portray him as inferior to Venetian men.
In persuading Brabantio to search for his daughter, Iago and Roderigo use language that paint Othello as a thief: "Look to your house, your daughter, and your bags! / Thieves, thieve!" Roderigo tells Brabantio that ". . . your fair daughter, / At this odd-even and dull watch o' th' night / Transported . . . / . . . / To the gross clasps of a lascivious Moor[.]" (I.i. 81, 117-121). Iago then skulks back to Othello while Roderigo joins Brabantio's search party.
Venice is in chaos as Brabantio, with swords, sticks, tapers, and guards, searches for the man who stole his daughter. Pretending to be a friend, Iago warns Othello of Brabantio's mission and tells him, "You were best go in." But Othello counters, "Not I, I must be found" (I.ii. 29-30).
Othello's desire to be found, his courteous response to Brabantio's harsh words, and his willingness to submit to Brabantio's dictates (I.ii. 60-90) negate Iago's and Roderigo's description of him and expose both characters as churls.

Iago uses bestial imagery to inform Desdemona's father of the elopement, casting Othello as a sub-human devil:

Even now, now, very now, an old black ramIs topping your white ewe. Arise, arise;Awake the snorting citizens with the bell,Or else the devil will make a grandsire of you

And again:

you'llhave your daughter covered with a Barbary horse;you'll have your nephews neigh to you; you'll havecoursers for cousins and gennets for germans.

And again:

I am one, sir, that comes to tell you your daughterand the Moor are now making the beast with two backs.

Each of these calls to action allows Iago to plant an image in Brabantio's mind of Desdemona's contamination through her marriage to Othello. We have yet to see Othello, and Iago's language feeds into stereotypes the Elizabethan audience might hold regarding a Moor. Coming from Iago's own lower class language and personality, the news must strike the Senator as an affront in both speech and content.
Roderigo, a more genteel young Venetian, offers a contrasting interpretation of the same news, sparing the lasciviousness of Iago's descriptions:

your fair daughter,At this odd-even and dull watch o' the night,Transported, with no worse nor better guardBut with a knave of common hire, a gondolier,To the gross clasps of a lascivious Moor--If this be known to you and your allowance,We then have done you bold and saucy wrongs;But if you know not this, my manners tell meWe have your wrong rebuke. Do not believeThat, from the sense of all civility,I thus would play and trifle with your reverence:Your daughter, if you have not given her leave,I say again, hath made a gross revolt;Tying her duty, beauty, wit and fortunesIn an extravagant and wheeling strangerOf here and every where

To Roderigo, the offense lies not only in the racial coupling but in the way in which Desdemona flouted the social norms of the upper class Venetian society, choosing to run off with a declasse mode of transportation, to an outsider with no claims on Venetian society, and in disregard of her duty and obedience to her father.
Iago offers a base and animalistic picture to outrage Brabantio's imagination; Roderigo offers a rude and non-Venetian one. Both offend Brabantio as a father and Senator.
When we finally see and hear Othello, we find him eloquent and elegant, bearing himself with as much or more control and dignity than the Venetians. He offers no resistance in his arrest, speaks rationally in his defense, and welcome's Desdemona's own testament of her free will. In these early speeches, he contradicts the prejudiced portrait Iago and Roderigo give and fill the imagination not of bestial images but of wonder and enchantment.