I am using Update panel in my custom webpart. I need to show the default sharepoint 2010 Loading message when user click on button and postback event fires.
If you are looking to use the SP.UI.Notify (http://msdn.microsoft.com/en-us/library/ee658473.aspx), take a look at http://bramdejager.wordpress.com/2011/03/21/using-sp-ui-dialog-and-sp-ui-notify-from-code-behind/.
Sounds like you are looking for the SPLongOperation. Please have a look here for instance: http://www.sharemuch.com/2010/01/22/displaying-sharepoint-2010-processing-page-during-your-long-running-custom-operations/
You should use SP.UI.Notify.showLoadingNotification()
you can use updateprogress control inside updatepanel with default waiting html content for SharePoint. Please see the link for details - https://sharepoint.stackexchange.com/questions/2813590/ajax-loading-icon-with-updatepanel-postbacks