How to align the buttons at the center of the screen in android

I have two buttons. I want to align them at the center of the screen,. I tried doing.. but i'm not able to do it.

fragment_main.xml :

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.assignment_1.MainActivity$PlaceholderFragment" >

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button1" 
        android:id="@+id/button1"
        android:onClick="Display"/>
    <Button
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/button1"
        android:layout_toRightOf="@+id/button1"
        android:text="@string/button2" 
        android:id="@+id/button2"
        android:onClick="opaquemessage"/>
    <TextView 
        android:id="@+id/display_mes"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"/>

</RelativeLayout>