In the root_folder/themes/mytheme/product.tpl i added data attribute to quantity input:
<input type="text" data-quantity="{$product->minimal_quantity}" name="qty" id="quantity_wanted" class="text" value="1" size="2" maxlength="4">
data-quantity="{$product.minimal_quantity}" is what you actualy need.
When users click on "add to cart" button they invoke function in root_folder/themes/mytheme/js/modules/blockcart/ajax-cart.js file (if not exists you should copy original file from root_folder/modules/blockcart/ajax-cart.js to path given above).
Function is called overrideButtonsInThePage. I made some changes to parse data-quantity from input for multiplying it to minimal quantity
i.e.
....
overrideButtonsInThePage : function(){
...
//for product page 'add' button...
$('#add_to_cart input').unbind('click').click(function(){
var input = $('#quantity_wanted'), // input
quantity = input.val() * input.data('quantity'); // parsing our quantity from data-attribute and multiplying by it
ajaxCart.add( $('#product_page_product_id').val(), $('#idCombination').val(), true, null, quantity, null);
return false;
})
...
Also you shoudn't forget to add similar code changes (i mean to add data-quantity attribute) to other parts of your shop (shopping-cart page, order page e.t.c)
After all changes done, when you set minimal quantity to your product (e.g. 50) users will be able to buy only 50 or 100 or 150 as it was needed.