If it was caused by a mistake, you can safely ignore them, or Mark as Fixed in Webmaster Tools. Google will continue attempting to crawl them, so they'll come back time and again, but continue to Mark as Fixed and they'll drop off eventually.
If the 404 errors are the result of a site redesign, you must try to 301 redirect each and every page to a corresponding existing page on the site. If that is not possible, make a useful custom 404 page to help users find exactly what they want (and make sure it returns a 404 header).
If any of those 50,000 pages have incoming links from other sites, make sure they are redirected to a corresponding relevant site on your page. If there is no corresponding page, redirect to a page which explains to the user what is happening ("this page no longer exists on our site, but something you may find interesting/useful is blah blah blah"). As a last resort, redirect to the home page.
Ryan Cramer: This may sound a little new agey, but "you can only take one day at a time".
A year ago I got put on a site that the developer did not manage the site and used a lot of black hat techniques that drove placement up, including creating keyword rich, but bad links (404). ;What I did was I marked them all as fixed and took one day at a time to find out what the real issue was. ;I doubt you are getting 50,000 per day.
It was an hack.... my website got hacked, and the hackers manage to add a folder with a scripted PHP page, just a single page, unfortunately we didnt came to know about that for a month and then we found that with that single page the hacker was able to create as many pages within our website.. just like a regular ;dynamic website... since we have deleted the folder, now m facing the issue these 404...... at they reached to 50k..... The only one thing I did now is created a 404 page, with NOINDEX attribute and redirected all 404 Errors to 404 page...
now is there is anything we could do?
Frank Ludriks: That's fine what you've done. If the hacker made links to pages that are now 404ing you can safely ignore those too.
Ryan Cramer: So sorry to hear about getting hacked... never ran into that issue before. I wonder if anyone has tried the Google Index->Remove URLs function in Webmaster Tools for something like this?
What is the best solution for 50, 000...... 404 Errors. in Webmaster Tools..?? 
